Aliens and UFOs Among Us
http://userpages.bright.net/~phobia/
Good photos, videos, and links.  Forum is small, polite, and respectful.  Be nice!

Aliens Central
http://aliens.monstrous.com/
Informative.  Lots of good information.  Great spectrum of topics in the forum here, and possibly “monstrous” at times:  Arrrgh!

Alien UFOs
http://www.alien-ufos.com/
Discussion forums: Fast paced and fun!

ATS
http://www.abovetopsecret.com/
Huge site - and a twist on news, too.