Xu Ji, Bora Mollamustafaoglu, and Gun Pinyo wrote a chess game for the RPi in Assembly Language, without bothering to use an operating system!
My two friends and myself have successfully written a chess game for the Pi. It’s a graphical game with a colourful chess board, so we interface with the GPU and whatnot, and we do it all in assembly without an OS running on it.
We actually wrote the assembler too (in fact this was the actual assignment for the project), but it was based on the ARM7 assembler so it’s probably compatible with official assemblers (although we have not tested this). It is due to the fact that we ourselves wrote the assembler (in C) and we only had 2 weeks for this that we only support a limited number of instructions in (our version of) assembly language. For example we are forced to use labels for variable names.
The game is written to replace kernel.img inside the OS and is about 15 thousand lines.