There are several sites around - such as Change.org and CREDO Action - that allow you sign and/or write petitions, but if you've tried using these sites, you have probably noticed that a lot of the issues are either presented in a partisan manner or don't even relate to actual bills being discussed. Signing these latter ones can be a waste of your time, but especially that of your representative. Signing partisan petitions may make you feel better, but is unlikely to make you much better informed on the topic.
So why not go straight to the source? POPVOX does just that. The 2011 SXSW Accelerator winner of the Social Media & Social Networking Technologies category gives you only actual bills pending before Congress, and a direct link to GovTrack.us to give you an objective, non-partisan perspective on them. In other words, it allows you to make up your own mind, rather than follow somebody else's lead.
And it only gives you two options on each bill: support or oppose. This guarantees that you send a clear message to your representative every single time you click. Better yet, you get to see charts of how and where opinions are trending in the 'POPVOX Nation'. All it needs now is more of us logging in and presenting our opinions.
It's something Richard Dreyfuss would be proud of: