I recently built my own cheap backup server using OpenSolaris and ZFS. I used my old SATA drives (6x400GB), a $75 motherboard and AMD Athlon X2 combo, 4GB of DRAM ($69) and an old tower case. I did add two SATA 5-bay hot-swappable disk bays ($110 each) so that I can easily replace/upgrade my disks. Once a week I update data from my main server (also Solaris) to the backup server using ZFS incremental snapshots.
My PC's and Mac's all mount their user directory from my main server, and I rsync my laptop every day. The main server also serves as a SunRay server so I do most of my daily chores on a SunRay. I run Windows inside VirtualBox and I rarely ever turn on my windows PC anymore (the Windows instance in VBox also mounts from my main server). Inside my main server I have 2x 1TB drives, in a ZFS mirror setup, for the user directories and 2x400GB for the OS and scratch directories (all drives are SATA).
I'm very confident in this setup, also because I can yank out my drives in under 30 seconds in case of fire. The only thing I still have to do is put my backup server in a different room from the main server - that is a todo project for the near future.