Well, depends on which machine. At present, I have three "multi-boot" machines.
One is my MacBook Pro with OS X Snow Leopard on the main partition, and Windows Vista Home Premium on the second. Apple's "Boot Camp" drivers include an HFS+ driver for Windows, so I leave my core files on my HFS+ OS X partition. (I also store photos, music, and movies on external HFS+ hard drives; and have a network share, whose file system is irrelevant.)
One is my PC, which dual boots Windows Vista Ultimate and Windows 7 Ultimate. Win7 is on a two-drive RAID-0; Vista is on a 100 GB partition on a 1 TB drive. I store my data files on the 900 GB partition of the 1 TB drive. (I even have the "Public" folder re-mapped to be on that partition for both OSes, and have some applications installed to that partition for both OSes, so I don't need duplicates of such things.)
The third is my netbook, which is in a really funky state at the moment. I have XP Home on one partition, and Win 7 Pro on a second, on the internal 16 GB SSD. Needless to say, it's a heck of a squeeze. I have Microsoft Office installed in both OSes, but it's residing solely on the XP partition. (Oh, and each OSes swap file is on the other OSes drive; which I auto-delete the other's on boot in each OS.) Data files are split between the two, because I'm lazy, and don't really have much need to share data there. I also have an internal USB flash drive with Moblin Linux installed. And when I get around to it, I'm going to figure out how to install HP's custom "Mi" Linux onto an SD card; so it'll be quad-boot. (Heck, if I can figure out how to "dual-boot" HP's Mi Linux from a single physical drive, I'll throw both it and OS X on the SD card. By default, HP's Mi will ONLY install to the primary internal drive, and will *NOT* dual-boot.)