How can I view/open an nrg file?

Isn't that Nero's proprietary disc image format (analgous to an ISO disc image)?

The only tools I know, other than Nero's own, are open source command line tools - there are several available for Windows (nrg2iso is/was one), or nrg4iso (old) or eNeRGy for Mac OS X. On a Mac, there are some unix apps that should also work.