There are dozens of ported applications (see the link in my signature) with more coming slowly but steadily. There are also lots of pure .NET applications, which work fine on RT once it has been jailbroken. In addition, there are a few custom-written applications for RT, the most important of which is probably
@mamaich's x86 dynamic recompilation compatibility layer (sort of like
Rosetta for old Macs) that allows running some (old and/or now-demand) x86 programs on RT without modification. Work on it appears to have largely stalled (although the current version is still available, and does work for some programs on RT 8.0) while the jailbreak for RT 8.1 is in development, but I'm confident it will pick up again.