You could help out coding Haiku OS (www.haiku-os.org) which is an x86 clean-room open source continuation of the once commercial BeOS being developed by a committed team. The emphasis is on native code, fast, lightweight and being super-responsive. You can have 10 videos playing at once (I don't know how much, but it's designed to never block a user's actions) and it doesn't bat an eye. It has binary compatibility with BeOS and new apps are being created for it all the time (avoiding the chicken-and-egg problem). There is a variety of them (some open sourced professional apps). It is not UNIX-like, but it has POSIX compatibility. Haiku has a journaling-file system which is fast and supports user-definable attributes (Be engineers brought a subset of it to Apple with them developing Spotlight and Microsoft had been working on a similar feature). Stack and Tile (haikuos.info/video/stack-and-tile.html) is a cool windows management feature unique because of tabs for window titles. The OS API is said to be the simplest and a joy to use.
Some of the languages available:
C,
C++,
Python,
Ruby,
Perl,
Lua,
Haskell,
Scheme,
Squirrel,
Yabasic,
bash (comes with a bash shell),
zsh
Some of the developer tools available:
Paladin (its own IDE), Git, Hg (Mercurial), GNU Compiler Collection, Vim, QEMU, Bochs
Some of the technologies used:
Multi-process WebKit browser,
OpenGL,
OpenAL,
SDL and Qt libraries,
Gutenprint and CUPS for printing,
FreeBSD network drivers,
Haiku Vector Icon Format
On the way:
An ARM port,
Clang with the already working LLVM,
OpenJDK,
Gallium3D,
package management,
more language bindings for the API
Included below is ways to contact, lessons and software sources:
freelists.org/list/haiku
freelists.org/list/haiku-development
haiku-inc.org
haiku-screencasts.blogspot.com
www.haiku-os.org/blog/darkwyrm
www.osnews.com/story/24945/A_Programmer_s_Introduction_to_the_Haiku_OS
haikuware.com (back online shortly)
bebits.com
There's also a wallpaper contest going on. I am not affiliated but I'd love to see more people get involved in this great OS! :)