ImageBurner is a front-end for the MacOS X "hdiutil" command-line tool. It allows you to easily burn disk images (such as ISOs) to CD-ROM or DVD-ROM using drag-and-drop, without messing with the command line or having to purchase Toast. There's nothing you can do in ImageBurner that you can't do in Apple's Disk Utility, but ImageBurner provides a simpler interface.