Are you looking just to back up data, like I do with my music and such, or do you want full, bootable clones of your drives?
The first option doesn't require partitioning if you're willing to keep the whole drive in FAT32, and has several freeware solutions on a Mac. As I said, I use rsyncbackup, which is a command-line frontend to rsync controlled by easy-to-edit text files. Other options include PsyncX, which appears to be a GUI frontend to both psync and cron, so it can even schedule regular backups for you. Also, Lacie Silverkeeper, which is packaged with LaCie externals but is also available freely from versiontracker. With any of these three free utilities, it shouldn't be more than an hour's work to set up data backups that run regularly.
The second option requires partitioning your external drive into a Mac Portion and a Windows Portion - the Mac portion being, ideally, at least the same size as your Mac drive. Once that's done, though, PsyncX and rsyncbackup can be used to make bootable clones, or you can use the venerable $5 Carbon Copy Cloner to schedule regular clones on the Mac end.
Finally, if you want the most flexibility and power in Macland, check out SuperDuper!, a $27.95 shareware utility that appears to be able to do, well, just about anything you could want backup-wise (including both of the above things, of course), and has a perfect 5.0 rating on versiontracker, so it's probably quite nice.
I'm sure judicious research could find similar progs on Windows, but I wouldn't really know where to start - is there a website with Windows downloads that's actually as well maintained and near-universal as versiontracker? Probably not.
ETA: Looks like Ghost may be a good option in windows. A bit expensive for my tastes, but I'm a minimalist. For the best combination of ease and power, Ghost + SuperDuper would probably work quite nicely for you.