Since iOS 4 Jailbreak is already available, it is time you pay attention to what apps will be compatible with the new environment.
So the German site hack2learn, has come up with a list, actually a spreadsheet of apps, their version, Current status, multitasking capability, and notes. The list is kept current by the members of the group.
So if you are the type who runs a jailbroken iOS on a compatible device, iPhone 3g, iPhone 3GS etc, it is a good resource.
Compatibility List on Google