Possibly the easiest way to register to vote (gottaregister.com)

To be honest, I got it from Barack Obama's AMA on Reddit http://www.reddit.com/r/IAmA/comments/z1c9z/i_am_barack_obam..., so the endorsement of the website comes from the POTUS himself.

I actually think it's disingenuous that they don't upfront that they're affiliated with the Obama campaign. There is a small bit of text, but it's intentionally hard to read and placed below the fold.

I'm pretty confident that if you use the site and enter your email address, you're going on their mailing list.