AviSynth is a cool app, but it can always be better; it's obvious enough to us anyway that there's way more than enough work to go around. We love doing it, but we also love fresh talent.
The most obvious way to contribute is by testing our software. If you think you've found a bug, please read Troubleshooting for instructions on contacting the development team. Comments on documentation and this website are always welcome, too.
Of course, code contributions are worth their weight in gold. (take that for what you will!) One way to familiarize yourself with AVS code is to read the Filter SDK -- if you can write a good filter, you'll definitely catch on fast when it comes to hacking the AVS core. (Nothing says you have to, of course; we need just as many solid filter developers as any other task I can think of). The next obvious step would be to download the latest source snapshot from our CVS repository on SourceForge. Coding is a black art at best and we're no magicians, so if it doesn't compile feel free to stop by the DoomNineForum and give us a holler. (Hint: download the Windows Platform SDK. All of it. Give priority to its /inc and /lib directories as appropriate.)
Ok, you've honed your expert video skills and finally got VC++ to behave. How do you contribute? Right now the best way is probably to get ahold of sh0dan or Wilbert -- once given the appropriate SourceForge rights you'll be able to patch the source tree directly. Cool, eh? We think so too; what are you waiting for?! :P