Developers wanting to test new versions of their apps (or users who insist on having more than one copy of an app hanging around in their Applications folder) can now make their lives much easier with a clever little app called rooSwitch.
The app’s purpose is to help you manage multiple data profiles for pretty much any app you have installed. Drag an app on to it, and it will quickly hunt down all the appropriate data, cache and plist files. These are collated as a saved profile (a .rooswitch file); then you can start having fun by making copies of profiles, or creating entirely new ones. This last feature is particularly useful for seeing an app as a new user would see it.
Describing in words what rooSwitch does is tricky, so your best bet is to watch the short explanatory screencasts that the author has produced; these make the whole idea abundantly clear.