Something like MHDDFS for OS X? I'm trying to have my Mac Mini take over all my server's functions (I'm recycling the server hardware into a Linux workstation).  Its pretty much all done, except for the file-serving portions.  Currently I'm using MHDDFS (FUSE).  In a nutshell, it lets you give it multiple directories (in my case on different hard-drives), and have them be combined in a directory.  (e.g., /media/storage0/ and /media/storage1/ are accessible via /media/storage_pool/  

What I would like to know is, does anybody know of anything that might act like this?

I know of NRAID/SPAN, but part of what worries me about that is (I think?) in SPAN the drives share a file-system, similar to a RAID level, which makes me very nervous.  (even though I keep backups)
Looks like it's available with MacPorts - https://trac.macports.org/browser/trunk/dports/fuse/mhddfs/Portfile