Here's a neat effort. The Human Rights Campaign is submitting an amicus brief in the gay marriage case that is due to be heard by the Supreme Court in April. They are trying to collect a million signatures by March 1 to go with it.
I think it's a great way to add your name to the record of a landmark case. OK, so you might get some junk email as a result, but that's a relatively small price to pay :)
http://www.hrc.org/
Scroll to the bottom to fill out info, click on "sign the brief" for submission.