In this roundup, we put four all-purpose, keyboard-based launchers to the test: Running with Crayons' Alfred (free, but £17/$26 for the highly recommended Powerpack option), Many Tricks' Butler ($20), Objective Development's LaunchBar ($29), and the Quicksilver Project's open-source Quicksilver (free).
We also take Spotlight into consideration, which is built into OS X and can do many of the same things. Each of these lets you open items on your Mac without knowing where they are (or even exactly what they're named), and they all share a number of other useful core features.
Get used to keyboard commands
With all these launchers, the primary usage pattern is the same. First, you press a keyboard shortcut, such as Command-Spacebar, to open a pop-up search window. (In some cases, you can also click a menu bar icon.) Then, you start typing the name of whatever you want to launch, and potential matches appear in a list. You can narrow down the list of matches by typing more characters or by using the arrow keys to select something other than the first item in the list. When the item you want is highlighted, you press Return to open it. For example, the complete sequence of keystrokes to open Mail might be Command-Spacebar, "m," Return.
Spotlight uses the Command-Spacebar shortcut by default, but you can change it if you like by going to System Preferences > Keyboard > Shortcuts > Spotlight > Search Results, double-clicking the keyboard shortcut next to Show Spotlight Search, and pressing a new key combination. Similarly, each of the non-Apple launchers has a default keyboard shortcut involving the Spacebar (LaunchBar uses Command-Spacebar, the same as Spotlight; for Alfred, it's Option-Spacebar; for Butler and Quicksilver, it's Control-Spacebar), but you can change this to whatever you prefer--just make sure your chosen launcher doesn't conflict with Spotlight. (If you want to use more than one launcher at the same time, that's fine too, as long as each has a different keyboard shortcut.)
What you can find and open
The word launcher implies that you'll be opening apps, and all the launchers (including Spotlight) prioritize apps in their search results. But Spotlight can find and open nearly anything on your Mac, including documents, folders, System Preferences panes, Mail messages, iTunes tracks, and contacts. In addition, Spotlight can search the Web (via Bing), perform currency conversions, show Dictionary definitions, and more. In Yosemite, Spotlight displays most results (including Wikipedia pages, movie trailers, maps, and so on) right in its pop-up window. (You can see the full list of search categories in System Preferences > Spotlight > Search Results, where you can also disable or reorder the categories as you see fit.)
Alfred, Butler, LaunchBar, and Quicksilver display and open most of the same data types as Spotlight, but offer much more configurability. For example, you can ask them to look at only specific files or folders, or perform specialized Web searches for, say, Twitter users or IMDB entries. All except Butler also have mechanisms for adding plugins in order to extend searches to include items such as 1Password logins and Transmit favorites.
All the launchers become smarter as you use them. For example, if you type "m" in an effort to find and open Messages, perhaps Mail comes up on the list sooner, so you arrow down to Messages instead. But the next time you type "m" (or, depending on the launcher, after a few tries), Messages comes up as the first choice.
However, Spotlight always groups categories as defined in its preferences. So if you have apps listed first, then no matter how many times you select "Many Merry Monkeys.docx" as your choice after typing "m," it'll still be listed lower, among the documents.
Extra actions
Although Spotlight has many talents, it lacks several key features most of the other launchers have. All the rest can run AppleScripts (not just open them in Script Editor), track your clipboard history and paste previous clipboards, and control iTunes (for example, pause or resume playback). Spotlight can play a particular track in your library and look up other artists, tracks, and albums in the iTunes Store, but it can't play an album by name. Butler and LaunchBar can (though not Alfred or Quicksilver).
All the launchers except Butler can perform calculations right in the search field. (In Quicksilver, you must type "=" as your first character to trigger the calculation.) In addition, all except Butler let you use Quick Look to preview a selected item without opening it. And all except Butler and Spotlight can open a document in an app other than the default one for that file type (for instance, opening a Word document in Pages), and let you type keywords to perform a wide variety of system actions--things like hiding the current app, changing your volume, or restarting your Mac.
LaunchBar can add an event or reminder from its search window. (Alfred can, too, with the addition of free third-party workflows.) LaunchBar can also fill in user-defined text snippets (including variables such as date and time) and emoji (by name), switch network locations and user accounts, perform any of dozens of built-in actions (such as compressing files, emptying the Trash, hiding an application, and performing text conversions), and execute Automator workflows and items on the system-wide Services menu. It also lets you perform other actions with selected files besides just opening them--sending files by email or Messages, running an AppleScript, and so on.
Quicksilver is perhaps the most modular of the launchers, with dozens of free plugins available to add features--like image and text manipulation--and allow integration with apps like Evernote, Google Chrome, iTunes, and Safari. It's also the most explicit about the separation between the item found (an app, a document, a contact, etc.) and the actions you can perform on it. The default action is nearly always Open (accomplished by pressing Return), but you can instead press Tab and view a catalog of other actions appropriate for the selected item.
User-friendliness and performance
While all four of these launchers are similar at a high level, each one has its own spin on the features it offers. Given my own tastes and mindset, I find LaunchBar to be the most straightforward and best designed of the bunch. You can use it almost immediately with barely any configuration, though it's easy to customize if you like. And it's both extremely fast and predictable, largely because it uses its own index rather than relying on Spotlight's index. (LaunchBar is also good at detecting multi-word names and camel case words, so if you type "FT," FaceTime is likely to be at the top of the list.)
Although Spotlight's usability is good and its range of features is impressive, its speed is not. Even on a fast Mac, it may take two or three seconds for the full list to appear, and during that time, elements in the list may rearrange themselves, increasing the likelihood that you'll select the wrong thing.
Alfred relies on Spotlight for everything except for apps, preferences, contacts, and Safari bookmarks, so enabling documents, folders, and other items in Alfred's list can slow it down, although you can work around this by typing "open" to start a search for these items. Alfred depends on the user knowing such keywords for various tasks--so there is a bit of a learning curve involved--but on the whole, it's a friendly and well-thought-out app.
Quicksilver uses its own index, too, but its performance slowed to a crawl when asked to index a big batch of files (such as everything in my user Documents folder)--and was even worse during its periodic index rescans. Although Quicksilver is simple to use for most tasks, it can be challenging to find your way around its preferences. And some plugins, like 1Password, are currently far out of date.
Butler frequently left me scratching my head. I didn't have any particular problems with performance, although it continuously searches when indexing large folders (such as your Documents folder)--descending into all the subfolders, no matter how deeply nested--just to show off its usefulness. My bigger complaint about Butler is that it's cluttered and oddly organized--it adds three system-wide menus in addition to the pop-up search window, and the preferences are often inscrutable.
Which one
For opening apps and documents, and searching the Web, any of these launchers will get the job done. Although Spotlight's preview feature is handy, it's less powerful and flexible than the dedicated launchers when it comes to other tasks.
Of this batch, LaunchBar is the most approachable and requires the least configuration, with Alfred coming in as a close second (but only with the optional Powerpack, which adds features such as iTunes controls, email searching, a clipboard history, and workflows). But if you already use--and love--a different launcher, the transition to a new one may seem awkward and unnatural. There are no bad choices, here, as long as they work for you.