You could develop in almost every coding language directly on the N900 itself (some might take more work than others to set up correctly, though).

I write all my C and C++ stuff on my N900, using vi to write the code and gcc/g++ to compile (I have never even used anything that can compile C code on my desktop/laptop(s), unless you count Scratchbox in a VirtualBox machine, for porting aircrack-ng and a couple of other things to the N900).

You can develop in any scripting language (Python, Ruby, etc) as well, and so long as you have the actual packages required to run the code, you should be fine there, as well.

Java might be trickier since getting all the OpenJDK stuff or some other java sdk and runtime environments working perfectly might be more of a problem - never tried.

Hell, if you felt like it and have an Android device, with some work you could develop Android apps, on the N900, and then run them from the N900 on your Android device over USB hostmode (I tried this a few months ago, and I set up the very primitive low level Android SDK tools succesfully, never could get ADB to connect over usb, but I didn't have too many opportunities to try.)

And I believe, at least in Extra-Devel, there are some actual IDEs for certain coding languages ported (monoDevelop or something like that comes to mind). I never looked at the IDEs because I like the aesthetic of coding on the command line. But there's also a handful of very nice code editors (i.e. text editors but with language-specific syntax highlighting/coloring). I personally recommend TxPad out of those, but others are also available (PyGTK, as mentioned in a previous post, is another one).

So the short answer is yes, the longer answer would be coding language specific, because for some languages, setting up the required tools on the N900 might take more work than it's worth.