This may or may not be quite what you're wanting, depending on the reason, but Disk Utility can actually do this for the most part.
If you have an existing optical disc you want to make an image of, select the disc in Disk Utility, then under the File menu select "Make image from [whatever]" (disk0, etc). In the Format section, one of the options is CD/DVD Master. This should create a disk image that is pretty much bit-for-bit the same.
You can then either double click it in the Finder to mount it, at which point it is treated more or less exactly like an actual disc, or use Disk Utility to burn it to a CD/DVD-R.
I've used this method to make fully-working copies of Windows install discs (completely legitimate--site license at the university I work at), so obviously it's good enough to at least create a bootable non-Mac-format disc. I've also copied (non-protected) video DVDs I made with iDVD this way, and the results work fine in a set-top player.
This might fail if the disc has something tricky (copy protection or such), but it's a pretty good tool.
You can create a disc from scratch doing the same as a New Disk Image, then formatting it the way you want.
If you already have a .cdr or .iso disc image and you want to use or burn it, the same holds true--double clicking it in the Finder mounts it more or less as if it were a real disc, and Disk Utility will burn it for you. Again, I've used this to burn bootable DOS CDRs using the images you download from, say, Seagate or Hitachi to test their drives.
If a Finder-mounted image isn't behaving enough like the actual disc for your taste for some reason, not sure, although there is at least one freeware image conversion/burning tool that I've used. Forgotten the name now, since it's not much necessary anymore, but a search on VersionTracker or MacUpdate will find it.