You'll find many challenges here: http://forum.lowcarber.org/forumdisplay.php?f=106

I often set a different goal for the bigger/ longer challenges if I feel the goal is unreasonable for my body. The lose 40 by Christmas challenge being a good example.... I joined with a goal of 20 pounds by Christmas, because there's no way this body is giving up 40 pounds in 4 months

The daily weigh thread is a good way to stay accountable, as well as the "first" challenge.