Mac OS X: If you want to be able to select your "default" browser on a link-by-link basis, directing some links to Firefox, others to Chrome, and so on, Choose Wisely allows you to do so.
After installing Choose Wisely you set the Choose Wisely application as your default web browser in your system preferences, instead of one of your already installed browsers. In Choose Wisely you can specify up to five browsers — here we've specified Safari, Chromium, Chrome, Firefox and Opera — and you'll be presented with that menu each time you open a link that would normally launch a new instance of your default browser. You won't be presented with the menu while browsing from within a browser however; the selection process occurs only for the initial link.
Choose Wisely is freeware and requires Mac OS X 10.6. Have a handy OS X application to share? Let's hear about it in the comments.