Valve is launching a crowd-sourced review system on Steam, and it'll let you give ratings to any game in your library on the PC distribution network. The beta goes live today.
You'll also be able to rate other peoples' reviews, like you can on networks like Amazon. Presumably this will work better than the infamously abusable user-review system on Metacritic, as Steam users can only review games that they've played. The system will also display each reviewer's playtime in parentheses next to their review.