HearFromYourMP.com

Another cracking idea from Tom Stienberg and the guys at MySociety (who also do (WriteToThem, TheyWorkForYou and PledgeBank)

If you enter your details, we'll add you to a queue of other people in your constituency. When enough have signed up, your MP will get sent an email. It'll say “20 of your constituents would like to hear what you're up to – hit reply to let them know”. If they don't reply, nothing will happen, until they get an email which says there are now 100 people; 200 people; 500 people – until it is nonsensical not to reply and start talking.

HearFromYourMP.com