the iMac uses a regular optical drive so you can go to the store and pretty much buy anything. If you want it to work with Apple software and be viewed as a "superdrive" you'll need to look for a Pioneer DVR-106 or 108
or get an aftermarket one that isn't nescessarily going to work with iDVD, itunes, or Finder burning. Most every burner is supported by toast and popcorn so I wouldn't really worry too much about it.

The MAJOR issue you need to keep in mind is that the iMac is very space efficient and has a optical drive that is around 7 inches long. I'd pull your out and measure it to be exact. Most optical drives on the market are at least an inch longer than this, so make sure you measure before you buy.