Browser Fairy allows you to set different browsers to open different links, rather than having to use the same browser for everything.
If this sounds familiar, you might be remembering Choosy, which worked as a Preference Pane and did a very similar thing. I have written about Choosy on several occasions, and MacStories covered it back in 2009! However, Choosy has not been updated in a long time, and I have happily moved to Browser Fairy, if for no other reason than its ability to import/export its own settings, so I can move those settings between Macs and back them up.
I generally use Safari as my web browser, but I make two exceptions: I use Google Chrome for any Google-related sites, and I use a Facebook-specific Fluid browser for anything related to Facebook, which I do to prevent Facebook from tracking what I do on other websites. (I started doing this before I started using Ghostery.)
This is especially important if you are trying to maximize battery life on a laptop, where Safari is far superior to Google Chrome (“Using Safari full time is like getting a new battery. Not even kidding.” — @bradleychambers), but need/want to use Chrome for some websites.
Bonus Tip: If you are looking at a website in Safari and need/want to quickly open it in Chrome (for example, if you come across a page that, for some inexplicable reason, still uses Flash), Bradley Chambers has an Alfred workflow to open the current Safari URL in Chrome. Browser Fairy also has browser extensions to send the current URL to another browser easily.
Browser Fairy ($5, Mac App Store) requires OS X 10.7 or later.
Thanks to Abby for first introducing me to Browser Fairy.