On windows - I would reccomdend using www.fs-driver.org
to access and write to ext2/ext3 partitions. On linux, you can def. read NTFS drives, but writing to them has always been sketchy and dangerous (unless DRASTIC changes have been made in 2.6.15... I know it says write support, but so far we're only able to modify files, not create/delete/change size)
Fat32 is a good solution for a shared partition, except for the 4gb file size limit - the biggest file you can store is 4gb, so no dvd rips.
If you wanted true universality you would share a drive over a network using samba or nfs - that way linux/mac/windows systems could all access and modify it.