Read/Write FAT32 from both native Windows and native Mac OS X.
No individual file larger than 4GB.
Read/Write NTFS from native Windows.
Read only NTFS from native Mac OS X
To Read/Write NTFS from Leopard: Install MacFUSE and NTFS-3G. You can install both with NTFS-3G for Mac OS X.
To Read/Write NTFS from Snow Leopard: Guide: Enable native NTFS Read/Write in Snow Leopard
(Be aware that some are of the opinion that enabling native NTFS in SL is unstable/unreliable, and favor the MacFuse/NTFS-3G method for Snow Leopard)
Read/Write HFS from native Mac OS X
To Read/Write HFS from Windows, Install MacDrive
To Read HFS (but not Write) from Windows, Install HFSExplorer