Windows uses the NTFS hard drive format for its hard drives. Mac OS X can read files on NTFS hard drives, but it can't write files to them, which is a glaring omission. Attempting to drag a file into an NTFS drive in Mac OS X will only result in your mouse cursor turning into an error sign. Since most Hackintoshes dual-boot Windows and Mac OS X, being unable to share files between hard drives can lead to problems with file management. Fortunately, you can enable NTFS writing on Mac OS X with relatively little trouble. Read past the break for more.
LAST UPDATED: July 28, 2012 (Mountain Lion support)
NTFS-3G is a Mac app that enables reading & writing to NTFS drives on Mac OS X Snow Leopard for free. You need to install the Macfuse driver as well, for NTFS-3G to work. Once you've installed both NTFS-3G and Macfuse, you'll be able to write files to your Windows hard drives flawlessly.
Unfortunately, development of the Macfuse driver has stopped, so there will likely never be any official support for newer versions of Mac OS X. In Mac OS X Lion and Mountain Lion, you'll instead have to install the OSXFuse driver, a replacement for Macfuse.
However, even with OSXFuse, NTFS-3G does not work fully in Lion and Mountain Lion; you will still get mounting errors every time your computer boots. To fix these errors, you'll have to install this patch for NTFS-3G.