Dropbox recently released a Chrome extension--currently in open beta--that adds a Dropbox icon in your Gmail compose window for easily adding sharing links to Dropbox files in email.
Here's how it works.
First, snag the Dropbox for Gmail (Beta) extension from the Chrome Web Store. You won't see an icon pop-up in your browser window once you install this extension as it doesn't require an interface outside of Gmail.
Next, just open up Gmail and click the Compose button. At the bottom of the new message window you should see a Dropbox icon next to the Send button.
Click on it and a new window will appear in Gmail. If it prompts you to do so you may have to sign-in, but most users should be able to start using the extension right away.
With the new window open, you are free to choose whatever files you want to add to your email. Dropbox breaks down your files into three sections: recent files, all your files (called Files) and your photos. The extension doesn't allow you to share links to folders, only single files.
Once you find the file you want, click on it, and then click the Insert link button. A link to your file will then be added to your email message. If your recipient is also using Gmail they will see options to download the linked files to their desktop or save them to Dropbox. Users of other email programs (such as Outlook.com) will have to view the files on Dropbox.com before taking action.
In my tests, email messages sent with Dropbox links took a little longer than usual to reach their recipient. But that may be just an issue with the beta version of this extension that will hopefully improve over time.
Nevertheless, for hard core Dropbox and Gmail fans this is a great extension to merge the two services into one productive package.