If you're struggling to squeeze additional HDDs into your current chassis, take a look at Lian Li's latest rack mount kit, the EX-H33.
The rack takes up the room of three 5.25" bays, and Lian Li assures us it'll fit nicely into the front 5.25" panels of most modern chassis. If you're out of hard drive bays and have 5.25" bays to spare, this one could come in handy.
The EX-H33 can house three 3.5" HDDs in its aluminium frame. There's a front-loading door (with a key for the security-conscious), and individual handles for each HDD bracket.
On the back, there's a PCB that'll allow "most" modern RAID configurations with hot-swap functionality and a 120mm fan to keep the disks nice and cool.
On its official website, Lian Li still lists the EX-H33's ETA as end of June '08, but this one should start to appear any moment now at an MRSP of $60.
Official product page: Lian-Li.com