Here are some of the wikis on Wikia I've taken an interest in and recommend visiting or contributing to:

• Pen Pal Wiki: I founded this one. It needs a large user base, more contributions, etc ;)
• Open Source Software: Found this one on the list of candidates for Collaboration of the month and immediately voted for it. I think it's got great potential to be the most useful tool out there. If it could list all open source software it would be a great tool to search for open source software, it just needs a great deal of collaboration to reach such a goal.
• Computing Knowledgebase: Another one which just needs lots of collaboration. At the moment it doesn't really have anything to distinguish it from Wikipedia, but if the appropriate content was uploaded it could.
• Search: This wiki aims to make some sort of search engine which provides results based on user page ratings (or something along those lines). It's a great idea, but unfortunately the wiki has become neglected and (apparently) the searching software/databases have been taken down. However, there seems to be some renewed interest in it (although not much in the way of contributions/action)

That'll do for now, but there's tonnes more I've come across over time, so expect this page to grow!