If you have limited space and a burgeoning CD collection, this storage hack from the Ikea Hacker blog might appeal.
The trick is basically to put two CD storage units back to back and on wheels, so you can rotate the new unit to get to the CDs at the back. They've used an old Robyn CD shelving unit from Ikea but there's no reason it wouldn't work for any pair of CD shelves.
As long as you don't mind having to swivel it to see the CDs at the back, this is a nice space-saving idea.
Limited Space? Just Double up the Storage [Ikea Hacker]