Of course the legendary Rooster Teeth had to be on this list. They are most famous for their machinima series Red vs Blue, and are some of the pioneers of the machinima community. This is just their YouTube channel, but they post all the videos that they post on their site here as well. If you haven't seen Red vs Blue, you MUST check this out. And if you haven't, they have many more videos that you may not have seen from them.
Some more machinima creators, responsible for the CS:S machinima series Leet World. More recently they made a live action series called Web Zeroes. Both of these series are worth checking out, although Leet World definitely got stale near the end. I've only watched a few Web Zeroes episodes, but they were certainly funny and worth a look. As of right now, they are not making any new content.