A few great bicycle blogs:
Streetsblog.org - By turning transportation reform into a daily beat, this blog became a real force for change in NYC.
Streetfilms.org - Video blog that highlights best practices in transportation planning/policy. Folks in dozens of cities have used these videos to advocate for bike lanes, better transit, and pedestrian-friendly changes.
Streetsblog.net - Network of 200 or so transportation reform bloggers, connected in ways that lead to info sharing and mutual success.
UncivilServants.org - Place where people can map parking permit abuse by city officials. The site got a ton of attention and resulted in widespread reforms of the 'free parking permit' system (including a 30% cut in the number of permits).
Livable Streets Groups  - Community and toolbox for transportation reform activists. This is in Beta right now, and we want the mature version to be amazing.