First, it looks like xover_exchange is gearing up for another round. As the name suggests, it's a crossover-oriented exchange ficathon based on a list of fifty fandoms. Last year, the list of eligible fandoms included BSG, Criminal Minds, SG1, SGA, and Supernatural. This year's list hasn't been announced yet, but I expect most of those fandoms will show up again. (The only one that wasn't written last year was BSG, but I have my fingers crossed that enough people offered and requested it to win it a place on the list again.) If you're interested, there's a poll going on now to determine what new shows should be added to this year's list. The leading contender at the moment is White Collar. You might also want to check out last year's sign-up form to see how that worked. It's a bit long, but it does a good job of providing writers with lots of info on what kinds of stories the recipient would like to see without being unreasonably restrictive.
Second, scifiland has just started a new round and is looking for new members. If you've been thinking about trying out a land comm and don't know which to go with, it's a good one to start with: it's well-organized, and you can use material from nearly any science fiction show, so you don't have to pick just one. (Assuming you have the same sorts of problems that I do with trying to choose favourites. Have I mentioned how much trouble I have choosing a favourite anything?)