Hard disk drives these days are cheap. Too cheap, in that while we love paying 30 cents/GB, the reliability is getting pretty poor. Doing backups, especially automatic backups is a must, but what about RAID?
One of the problems with RAID, at least RAID-5 is that you need to have 3, and ideally 4 or 5 drives in a machine. That's a lot of drives, a lot of power, a lot of heat, a lot of noise. And many machines only have two IDE controllers so they can barely do 3 drives and can't readily do more even if they had the slots and power for them.
So I propose a software RAID-5, done over a LAN with 3 to 5 drives scattered over several machines on the LAN.
Slow as hell, of course, having to read and write your data out over the LAN even at 100mbits. Gigabit would obviously be better. But what is it we have that's taking up all this disk space -- it's video, music and photos. Things which, if just being played back, don't need to be accessed very fast. If you're not editing video or music, in particular, you can handle having it on a very slow device. (Photos are a bigger issue, as they do sometimes need fast access when building thumbnails etc.)
This could even be done among neighbours over 802.11g, with suitable encryption. In theory.
Not that there aren't some major issues to overcome. The machines must be on most of the time. (A single disk can be taken out of a RAID temporarily, and thus a single machine hosting one disk can be turned off or rebooted, but not for long periods.) If you lose access to two disks (or your LAN) you can't get access to the data. And it's going to use a lot of your network capacity, though gigabit networking is starting to get cheap. And the idea gets better...