SpanBurner is a simple app for Mac OS X that can burn a set of folders and files to multiple CDs or DVDs. It creates conventional files and folders on the discs it writes, so that they can be accessed from any computer system (including Windows) that can read standard discs, without any special software. It's ideal for backing up folders that won't fit on a single CD or DVD, which is otherwise difficult to do on OS X. (Some fairly expensive disc burning or backup utilities can do it, but they cost many times what SpanBurner costs.)
SpanBurner doesn't replace backup utilities (see caution box, below). Rather, it's for when you really need to write discs that follow normal file-system conventions.
SpanBurner is very clever about splitting folders while keeping the folder hierarchy intact, but possibly duplicated on different disks. But it won't split files. If a single file is bigger than will fit on a disc (4.6 GB or so for DVDs), it stops with an error.