Wales has in the past asked on Twitter for any tipps on geo ip matching visitors with local representatives/districts. Something like this would be "THE" golden tool for any grassroots campaign. I truly hope they/EFF are working on something like that.
It requires you to enter your zipcode manually, but it seems like a trivial issue to use ip address instead.
The complication comes from the fact that IP addresses aren't necessarily accurate enough to get a person located down to the exact address, which is what you sometimes need to identify your congresscritter.
That, plus the fact that visitors might be at work or otherwise away from home, making IP matching irrelevant.
Otherwise, the EFF tool is superb. I used it to enter my address, and was given a form letter that I could simply sign, along with the names, addresses, and phone numbers of all my representatives.
