It depends on how you partition it.
The Mac can read FAT32 without a problem. The Mac will read NTFS, but cannot write to it. There is a program called MacFuse, which can read and write to NTFS drives.
The old Linux ext2 is supported by both PCs and Macs. This gives you an HFS+ like file system (supports userids, ownership permissions, the works) that is shareable between all of Mac, PC and Linux.