There are a lot of them going on the web. I found quite a few through Epic Edits but I've seemed to missed the boat. The December challenge, a portrait for each day of the month sounded well worthwhile. This challenge was set up by Trevor Carpenter, here's his link.
Trevor has a community photo challenge on at the moment. Check out his website for more details.
There's also Ben Spark, who particiated in the self-portrait with your camera competition. He's just completed his taking a photo everyday for a 1,000 days (that includes uploading a photo a day!!) and has committed himself to another 1,000 days.
On a smaller scale, there's a 365 plus 1 group on flickr.
I'd really like to get involved in something like this. I'll keep my eye out for them.