My guild has been running a UFO challenge for the past couple of years, very similar to the one described by Onebyone. I don't participate so I don't know all the rules, but I think finishing a potholder or something equally small doesn't count. The challenge leader announces at the meetings how many UFO's have been completed each month. Also I don't think there's a clear definition of what a UFO is, so one person might consider a project that just needs a few binding stitches to be a UFO where another might include items that are still in the preliminary planning stages. For the first year's prize, the leader donated a very nice new sewing machine that she had won and didn't need.