JBit2 - Edit, save, run and debug assembly (6502) programs on your mobile phone.
JBit is essentially a 8-bit microcomputer (i.e. home computers popular during the 80s) and is NOT going to be easy to use, but with time and dedication, tinkering with it can be rewarding. The ideal user of JBit is probably a commuter who has already some experience programming in a high-level language (e.g. Java or Basic) and would like to have a go with low-level programming.
The JBit2 versions have pretty much the whole MIDP2 Game API mapped, so, in theory, you could use JBit to develop games on your mobile phone.