There are others but http://completionist.me/ works the best for me. Shows the averages and also has things like "this many games are under 5 percent completion" so I can go find something I booted up once and forgot about.
I also use steams category system to then tag some of those into a backlog category, although that doesn't help as much
I also use steams category system to then tag some of those into a backlog category, although that doesn't help as much