Define neat.

I started by writing some image processing code, then writing a couple apps with
some opengl stuff (see also cl-opengl), then gui stuff (see
LTK and the
ABLE editor).
Most of my effort lately has been focused on finding better ways of documenting and managing CL packages. This mostly involves
redesigning LibCL, as well as some private sketches of replacements for ASDF.
Tim Daly and friends could always use help on the open-source computer algebra package
Axiom and its forks
OpenAxiom and
FriCAS. These have a very mature codebase, but are undergoing changes in documentation, GUI, source compilation, etc. I believe they wanted to add a lazy streams system; that could be layered on top of clazy or perhaps funcallable objects (things I didn't understand back when the idea first came up).
Others do AI, music, games, compiler design, etc...
So what are your interests?