You want something like this: http://www.fentek-ind.com/FootPedal.htm#afs3usb
There are cheaper variants, and ones with fewer pedals, but keep in mind your feet exert more force on things than your hands, so you want to get something well made that will stand up to dirt. To put it in perspective, cheap flight simulator rudder pedals never last very long . A cheaper 1 button options is this one: http://www.google.com/products/catalog?q=computer+foot+pedal&um=1&hl=en&bav=on.2,or.r_gc.r_pw.,cf.osb&biw=1557&bih=852&wrapid=tlif133188445718610&ie=UTF-8&tbm=shop&cid=12790204644764204510&sa=X&ei=uvFiT7rjEuSY2wWtg_XsCA&ved=0CJMBEPMCMAE#
That thing basically works with software you can bind various keyboard commands and macros to do whatever you program it to do. The more spendy ones usually have better software and can do more things. You can also use them as a shift key to enable secondary functions on a mouse or a g15 style keyboard too in some cases, which comes in handy once you get used to it.
Edit: if you have wierd prehensile toes like I do, the other cheap option is get a second USB mouse with fully programmable buttons, (microsoft intellimouse or the like), put tape over the mouse sensor, then chuck it on the floor somewhere. You could even program all the buttons to trigger push to talk, then you just step on it. I find that works best with the mouse backwards and the toes on the front, or with a really flat mouse like one of those microsoft arcmouse thingies. Edited, Mar 16th 2012 1:03am by Kaolian