I like a fair amount of the content being produced on Geek and Sundry (http://www.youtube.com/user/geekandsundry
), including The Flog and Written By a Kid.
I stumbled across a series on YouTube called Squaresville which is pretty decent (http://www.youtube.com/user/squaresvilleseries
I like John and Hank Green and I regularly watch their videos on vlogbrothers (http://www.youtube.com/user/vlogbrothers
), though it's not a series per se. I also found The Lizzie Bennett Diaries, which Hank Green produced, to be kinda fun.
I recently helped a friend do some pre-production and script work on a pilot for a web series that she wants to make. She's filmed the pilot and is doing the post-production work on it now. I don't entirely agree with her creative direction but it's her project, not mine, so we'll see how it turns out. I'm also getting ready to help another friend produce a pilot for a non-fiction web series.