The list is not so long in fact. Note that opinions given below are just my point of view, not the ultimate truth.
1. Assembler - for advanced users, accelerating other languages in places where CPU should be pushed over the edge. Compiled.
2. C - The base general purpose language. Compiled.
3. C++ - The same as above, higher level of abstraction, not suited for low-level stuff (drivers, libraries etc.). Compiled.
4. AmigaE - Can be considered as some "nicer version of C" without strict type control and with object oriented programming elements. General purpose, building applications mainly. Compiled.
4. Perl - scripting and text processing. Used in the SDK for things like automated header files generation. Interpreted.
5. Python - considered a script language, often use for scripting inside applications, can be also used to write applications. Covers similar application areas as Lua. In my opinion a bit bloated, on the other hand has lots of useful libraries. Interpreted.
6. Lua - It seems it is the main competitor for Python on the MorphOS platform. More lightweight, probably faster. In my opinion easier to learn. Currently unfinished. When done it will be usable as scripting language (system-wide and inside applications) and rapid application building language. Interpreted.
7. ARexx - scripting and controlling applications. The main disadvantage is ARexx is only available as 68k binaries and is not distributed with MorphOS. For controlling programs (its most popular application), can be replaced with Python, and soon with Lua. Can be used to create applications, but it is rather cumbersome. Interpreted.

There are more languages (Pascal for example), but in my opinion not ready to use on MorphOS (well, Lua is not ready too, but it is actively developed).

[ Edited by Krashan on 2010/12/9 8:48 ]