For a simpler solution, how about a Time Capsule (still a single drive, though) or a locally attached dual drive enclosure? But that's only gonna work well for one computer...
For more than one computer, you could try what I've done - FreeNas on an old machine. I currently have FreeNas 8 installed on an old Dell Optiplex GX620 with 4 hard drives installed (2x 1TB, 2x 500GB = 3TB total). My important data lives on the two 1TB drives, set up as a mirrored RAID 0. For the two 500GB drives, one is a Time Machine backup drive for my MBP, the other is a media share for my PS3 (ripped movies).
DISCLAIMER: FreeNas is just that - FREE. However, it is NOT the easiest thing in the world to get going. Getting a simple set up going is pretty easy - install the drives, install FreeNas to a flash drive, boot from there, share and go. Its some of the more intricate things that are dicey; namely, Time Machine. Even though its supposed to be a simple software switch to get TM working, I had to do some extras to get my MBP to play nice with the FreeNas server.
Also, even though FreeNas will run on just about anything (I think the specs are like 133MHz Pentium or better and 128MB RAM), you'll need to consider a few things:
1) Many old computers can't/won't boot from USB. Realistically, you'll need a Pentium 4 or better.
2) Many older computers are NOT energy efficient. i.e., You're sucking down a lot of juice for no huge benefits.
3) Many old computers only have room for 1 or 2 hard drives. My GX620 technically only has room for two; I have the other two installed by some of my own doing.
All of this leads up to my overall FreeNas solution: As soon as I get my tax refund, I'm buying an HP Proliant Microserver. This is a small business class server with a low power AMD Neo processor. Good news, starts at $349 from HP; Better news, you can find last years model BRAND NEW on eBay for less than $250 all day long. It supports up to 4 SATA hard drives, an optical drive, USB and eSATA outside the box, up to 8GB of RAM, and can run most any server OS you want (save for OS X Server, of course). I plan to stick with FreeNas for now, but have considered Windows Home Server 2011, which has an add-in for Time Machine. Its only $50, but then again, FreeNas is... FREE.
Sorry for the long post! Moral of the story: There are ways to do this that are cheaper and more robust than the Drobo!
LINKS:
FreeNas HP Microserver HP Microsever on eBay
EDIT: Oh, yeah - don't forget that if you DO decide to go the FreeNas route, you can always remove the drive from your external enclosure to use it with a "server" of some sort. But that will void the warranty of your external. But backup your data first... FreeNas uses its own partition scheme, so you'll have to reformat the drive before you can use it.