I researched NAS enclosures a while back, and came to the same conclusion. They either seem very expensive ($200 or more) or are non-standard and require some sort of driver to be installed on each system.
I have a 1TB NAS terastation from Buffalo that works great. It's rather expensive, but this is mainly due to the fact it has an on-board RAID controller that gives me extra configuration/redundancy options. But for what it cost these days I could have built a cheap computer with SATA drives and raided them together for the same affect.
If I was going to do it again, I would buy a simple USB enclosure, plug it into a computer that I leave on all the time, and simply share the drive on my local network. This might be your best bet, it's a snap to setup, especially if all of your computers play nicely together (all Windows or Mac etc).
