Is your project in an in an agile development context of user stories? Do you want to let developers estimate their work? Then, just play the planning poker. The idea behind planning poker is simple. Individual stories are presented for estimation. After a period of discussion, each participant chooses from his own deck the numbered card that represents his estimate of how much work is involved in the story under discussion. All estimates are kept private until each participant has chosen a card. At that time, all estimates are revealed and discussion can begin again. If you are interested to do this estimation on-line, then go to planningpoker.com, a free hosted software provided by Mountain Goat Software.