Ofcourse there is also the Ivibe
http://www.ivibe.com/default.htm
Version 3 is almost ready for sale according the the forum news.
This product does 2 things:
1) Same as the buttkicker , so react to low freq sound. But then with 5 or 6 "shackers/kickers"
2) If the game is supported then it will do some more inteligent stuff like forcefeedback.