To create an ISO image in OS X, use Disk Utility to burn a new image (from either the folder/raw files or from a dmg) choosing “CD/DVD Master” as the Image Format. That creates an image with the .cdr extension. Just rename the .cdr file extension to .iso and there you have it!
If you already have a DMG and want to convert it to an ISO, you can also use hdiutil to do so:
hdiutil convert discimage.dmg -format UDTO -o output.iso
You can burn existing/created iso's to a bootable disc by...
Inserting a blank CD/DVD
When the Finder dialog appears, select "Open Disk Utility" (make sure "always do this" is unclicked).
After Disk Utility opens, drag the ISO image file to the lower empty portion of the left-hand pane where disks and volumes are displayed in the Disk Utility dialog (unless it's already there) and select it/highlight it.
or... From the File menu, choose Open Disk Image and select the ISO to be burned (it will now automount the Disc). In the list of volumes, you will now see an item representing the ISO file. Select it.
Click the "Burn" icon at the top-left of the Disk Utility dialog.
BTW, you might intuitively think that opening Disk Utility and executing steps 3 & 4 and THEN inserting a blank CD/DVD would provide the same results but this seems to sometimes not be the case. When I tried it that way I invariably didn't get a bootable disc.
How to burn Wii iso's
As above, use Disk Utility to Burn the ISO. Remember to always set the burn speed to 1x or 2x and to set it to verify the disc after burning.
Note: At 2X it takes about 35min in total for a game that uses the whole disc (34min to burn and 1min to finalise) and remember this also works for Gamecube games.
For Windows compatibility
I think there is some confusion between naming something ".iso" (which, if a .cdr image, is then burnable in Windows) and the actual ISO-9660 *filesystem*, with or without the Joliet extensions, which is an actual burnable, mountable and useable VOLUME in Windows. If you do the above with hdiutil or as before that, use the ("CD/DVD Master") from Disk Utility you end up with the resulting burned CD having only 1 filesystem - HFS+ (Mac OS Extended) that is not very useful in other OSes.
For Windows compatibility (a CD or volume you can actually mount) you want the ISO-9660 filesystem, with or without the "Joliet extensions" to the ISO specification. Or maybe you want UDF for DVDs. What if you could have HFS+, ISO-9660, Joliet, and UDF all on the same image - and have it be smaller than the file created by the above Terminal commands? Well, you easily can - here's the quick and dirty how-to:
Drag the .dmg file (the CD, DVD or volume of which you want to use in Windows or Linux) to your Desktop.
Then go to the Terminal and type:
hdiutil makehybrid -o ~/Desktop/[outfile] ~/Desktop/[source]
Press Enter.
[outfile] is the resulting image. Leave off .dmg, .iso, .cdr, etc. from [outfile]. Include .dmg for [source] if it is a disc image. [source] can also be a folder.
That's it! The "example.dmg" which you dragged to your Desktop is just any name for the resulting image file. You can rename later, so don't dork it up - leave out spaces, symbols, etc. The correct filename extension will be added only if it isn't part of the provided name. Also, the path is case-sensitive. The currently logged-in user likely does NOT have a ~/desktop, but DOES have a ~/Desktop directory.
...but wait - it gets better! [source] can be the name of a FOLDER! It does not have to be a .dmg image file. Just get the path correct (does not have to be ~/Desktop if you want to make an image from a folder elsewhere, or a disc image elsewhere) and "viola" - you get a perfect hybrid disc image file useable in virtually *every* modern OS and can even be a hard-drive! Heck, for all I know can be a very large database file by itself...try it and see. The advantage of a hybrid system is that creating a cross-platform ISO9660/HFS+ hybrid has a minimal overhead when compared to a single filesystem image.
DMG Converter can create ISO filesystems, but it cannot CONVERT images to ISO filesystems, it can only make new images using it. So if you mount the image first and then use DMG Converter on the mounted volume, "converting" the disc image is the same as "making" a new image from the mounted volume.
