Nice little article on how to Build your own Drobo-Replacement based on ZFS. Of course, there have always been ways to replicate what a Drobo does in Open Source, and lots of criticism about the high cost of the Drobo, but the fact is that I have one and recommend it freely (though if DR wants to pay me I'll pimp it out more!). You are paying for the ability to plug in a black box (in the literal sense as well as the software sense) and have it Just Work and do what it should when you pop in some new drives, instead of mucking around at the command line to expand your raid sets or deal with resyncing.
If you can't afford the drobo though it's a good article on how to set up something that does mostly the same thing (in some ways better, in some ways worse).