I ran into this bag in a sport shop and never seen such a bag in a gym or anything but had incredible fun with it and it actually hits you back and makes you practice defense which is what most bags don't do (however it doesnt extend high):
http://www.amazon.com/Everlast-2260-Freestanding-Reflex-Bag/dp/B001FX5L2S/ref=sr _1_1?ie=UTF8&s=sporting-goods&qid=1258158049&sr=1-1

btw would you guys recommend one that is crazy movement wise or slower for more movement type training rather than hit training compare and contrast the two below:

http://www.youtube.com/watch?v=1erjx34e2M0&feature=player_embedded

http://www.youtube.com/watch?v=Yp14kAfW1og

I think if they existed in bruce Lee's time he would also use it for elbows no?

There is also a combination of a reflex and striking bag (also allows you to hit body) called the reponse bag but I dont know if it would be fast enough?
http://www.martialartssupplies.com/kingcobrafreestander.html

here is one where only the top moves not the entire rubber:
http://www.youtube.com/watch?v=DAOQ7jb9m34&feature=related

LEAN ON ME!

The reflex bag I got is from everlast and it is great esp for muay thai. When you keep it low about mid level you can throw roundhouses at it and alternate between teep kicks or sidekicks and flying knees at it all while it is moving and hard to hit. Great workout. Btw anyone tried mens inflatable punching bags before? how are they?
http://www.amazon.com/NET-Mens-Inflatable-Punching-Bag/dp/B001U0FFGI/ref=pd_sbs_ sg_49

http://www.fitness-equipment-superstore.com/p/blackline-ab-jab-punching-bag?utm_ source=Yahoo_Shopping&utm_medium=Shopbots&utm_term=www.fitness-equipment-superst ore.com&utm_campaign=1261305__Ab+Jab

here is a bag where you get to practice attacks form mount.

Punching from the guard.

You can do the same thing if you squeeze a heavybag between your legs.

well that one looks like speedbag so Im guessing it moves more stimulating head movement.