To update the information related to this rather old thread I can verify that even in El Capitan some external Zip 100 drives will work perfectly, with no additional software needed. First, however, my older style USB ZIP drive (P/N 04041100, model Z100USB, manu. 10/13/1999), a blue transparent case one that is boxy and has its own power supply and I think is technically second generation, is not recognized by my Mac Mini (late 2012) running El Capitan 10.11.4. Although you may find an older Iomega driver for early Mac OS releases it is actually a PowerPC version that, first of all, would require Rosetta (of course no longer included with OS X) and that driver is certainly far too old anyway. However the good news for people like me who still have a bunch of old Zip disks (PC and Mac formatted) with some data that we occasionally want to retrieve is that you can mount them and access their contents perfectly well with the "newer" style USB Zip Drives (mine is P/N 30897300, model Z100USBS, manu. 11/14/2002) that have the more streamlined, opaque blue cases (there is a proper name for that case style but I can't recall it). These newer drives are bus powered (only) and when I connect mine to a USB port on my Mac or a USB hub, insert a ZIP 100 disk, the disk is automatically and quickly mounted on the desktop. I see that you can buy even more recent USB ZIP 100 drives than mine, of the "modern" style, but they seem to me to be stupidly expensive for the technology they offer (about $160+) and it would really make no sense in my opinion to buy ZIP disks ($5+ per disk) when you can get 8 gig (or sometimes higher) USB thumb drives for that price.
I will also add that the better, modern linux distros (like Linux Mint) handle these newer USB ZIP drives perfectly well, and can mount both PC and Mac-formatted disks provided you have installed the necessary format drivers for hfs/hfs+, etc. In fact the older ZIP drive I mention here that doesn't work with El Capitan on a Mac Mini works fine in linux (and I would assume Windows, at least up to Win 7 if not beyond).