monline

"nrg4iso is a command line utility designed to extract data from a Nero Burning ROM image file (.nrg).
Nrg image files may contain various types of data (audio, iso9660,...) and nrg4iso will as development progresses be able to extract most of them."

The program is not developed any further, but a universal binary is available and it works just fine in OS X 10.5.x (and most probably in 10.6.x too).