here is the code that powers the sub-standard wiki on my current drupal code base. its quite easy to use, but just lacks doing revision control (which i intend to build on drupal's revision control) and backlinks. also a proper wiki syntax filter needs to be built. i still haven't decided on which one to use.
if you like to help me out building on it, let me know. i have limited time so maybe it might be a good idea to commit it to drupal cvs sandbox for people to play with.
you can find it here wiki.module
note, this is massively pre-release.