Little Monk (originally on usanetwork.com)
IMDb; USA Network (archived); Wikipedia; YouTube

Based on the series Monk. Wikipedia mentions some random Monk webisodes that I don't remember if I ever saw, but I definitely did see the 10-episode "Little Monk" miniseries, which streamed in 2009, during the eighth season of the show. It was about Monk and his brother Ambrose when they were kids. I don't remember it well now, but I must have thought it was at least a bit fun to watch at the time. It's no longer on USA's website, but there are at least five of the ten webisodes on YouTube. I guess the miniseries is also included as a bonus feature on the Best of Monk DVD.