Usually $100 elsewhere, the Buttkicker BK-GR Gamer is about $70 shipped "for a limited time" at fspilotshop.com.
Description:
http://www.thebuttkicker.com/gaming/products/bkgr.htm
"For workplace or home, the ButtKicker Gamer easily attaches to most standard center post chairs. Feel the low end without making the room loud. Turn down or turn off your subwoofer and still feel all the bass you want. Experience powerful bass even while using headphones. Ideal for apartment or dormitory use."
LINKY:
The FlightSim Pilot Shop - Buttkicker BK-GR Gamer $59.95 + s/h
I personally bought one to attach to my Playseats racing seat (with a little mod needed).
Description:
http://www.thebuttkicker.com/gaming/products/bkgr.htm
"For workplace or home, the ButtKicker Gamer easily attaches to most standard center post chairs. Feel the low end without making the room loud. Turn down or turn off your subwoofer and still feel all the bass you want. Experience powerful bass even while using headphones. Ideal for apartment or dormitory use."
LINKY:
The FlightSim Pilot Shop - Buttkicker BK-GR Gamer $59.95 + s/h
I personally bought one to attach to my Playseats racing seat (with a little mod needed).