If you want to access your Windows partitions, read and write on them, and do the same for your Mac OS partitions under Windows, then this tip is for you

NTFS pour Mac® OS X (shareware)

The Software provides full read-and-write access to your Windows Partitions.

MacFuse (freeware)

MacFUSE allows you to extend Mac OS X's native file handling capabilities via 3rd-party file systems.

For Windows:

MacDrive (shareware)

MacDrive is the only solution you need to share files between Mac disks and Windows.