SC2GG runs a weekly Starcraft 2 competition that anyone is welcome to join.
They do an NA one on Tuesdays and EU one on Wednesdays.
North America:
http://sc2gg.com/forum/index.php?showtopic=12879
Tuesday, October 5th at 8PM Eastern Time (EDT)
http://srv04.datona-1.com/~admin699/weekly/NA_rankings.html
Europe:
http://sc2gg.com/forum/index.php?showtopic=12831 (Last weeks tournament)
Tuesday, October 6th at 8PM CET (7PM UK time) will be the next one.
http://srv04.datona-1.com/~admin699/weekly/EU_rankings.html
Signups are open until about 5 minutes before it starts, and then brackets go up.
#sc2gg is the general IRC channel, #sc2gg-weekly for the weekly channel, both on Gamesurge.
Anyone is welcome to join (you don't need to hang around on IRC). Any skill level is welcome. Not everyone is Diamond (in fact many people are not).
Some games are streamed with commentary, and sometimes there are VODs done from replays after the games.
It's all in the name of fun, not a serious competition (although you do get points for playing and doing well, there's an overall leaderboard).
Usually there are at least 16 players per tournament (although NA gets more at the moment). The more players, the merrier. Feel free to sign up and play!
The only prizes (at the moment) are props and having some fun playing SC2.
Anyone who might be interested feel free to sign up (it's free and just casual). I play in the EU one, but usually we barely manage more than 16 players, so we need more numbers!