There's a weekly hard-surface thread. I haven't done it myself; however, a friend did and his modeling skills really improved. To add, the nice part is that other people are attempting the same models, so there are hints/tips/ref to look at throughout the thread if you need. I'm sure if you did all 50whatever weeks you'd…
Kind of depends on your skill level, but +1 for the Weekly Hard Surface thread. Will certainly test you in the beginning, but there's a lot of examples of people solving common problems, posting their wire frames, and other useful bits of info that will definitely help out. If you're completely new to modeling it might be…