One nice feature that Apple is starting to add to some of their application is multi-library support. For instance, if you launch iTunes or iPhoto with the option key held down you'll get a dialog box asking you which library you want to use. Well rooSwitch takes this idea to the next level by making it possible to have different "profiles" for nearly any OS X application. The profiles include preferences, caches, and even application data like the aforementioned iTunes libraries, all storied in a single .rooSwitch bundle. To use it you just drop the icon of the application you want to create a profile for onto the rooSwitch window and you can then create multiple profiles that you can easily switch between using the radio buttons. rooSwitch can even be used to backup application profiles and transfer them between machines or accounts. rooSwitch was originally to help developers test different scenarios, making it a good way to test (especially beta) software without losing preferences and application data.
rooSwitch is $14.95 and a demo is available. In addition, there is a free lite version that is limited to six applications (Mail.app, Address Book, Safari, iTunes, iPhoto and Stickies).