I'm trying to debug an application that was not created by me. If any of you have seen OllyDBG for Windows, that is what I am looking for but for the iPhone.

I've tried iphonedbg, but it always comes up with the error "process error!!!". I can only assume that this debugger is old and apple has patched it, or it simply isn't compatible anymore.

Does anyone know of any tools I can use to debug/reverse engineer an application on the iPhone?

you can run and attach a debugger right in the phone but I'm unsure how to do it exactly, and some apps have safes in them to force the debugger to stop once its on the appstore.

Best choice you have for reverse engineering is using IDA Pro, but it yields the assembly code for the app only.

You could try reading the console logs using xcode's organizer or iphone configuration utility