iVibe.com is selling tactile feedback sits for $239.95 :

The TFSU itself is a thick and durable seat cushion made of the highest quality fabric laminated polyurethane foam. It will fit in the chair you already use right now. Contained within the TFSU are 6 powerful electromechanical actuators, arranged in 3 pairs: one pair in the back section, one pair in the left leg section, and one pair in the right leg section. All three zones can function independently or in unison. By having three independent zones, games that support intellivibe can render things such as vector based gforce, acceleration and braking cues, direction-based incoming flak, and just about any other thing you can think of.

You can use their SDK (not yet available) or let the system create effects based on the sound of your application :

AudioSense digitally samples the sound effects from your computer as they are generated in real time. It then uses a digital signal processing algorithm (that is uniquely customized for each specific game or simulation) to tell the TFS2 how to generate useful tactile feedback based upon this real time digital audio analysis.

This with the tactile vest makes a nice feedback setup =)