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.