www.ideasforseattle.org
Very cool website implemented by new mayor of Seattle, Mike McGinn.
I haven't signed up, but it looks like anyone can (possible downside).
Although I think the residents have spoken!
EDIT: was just looking around and noticed that there was a completed tab. Pretty cool. Many of the items here were website related, but a few already completed.
There is also the accepted tab, where items have either been started or a now being planned for.