GitX is a Git history viewer (like gitk) for OS X.
Git is a popular Distributed Version Control System originally created by Linus Torvalds to coordinate work on the Linux kernel.
GitX is a gitk like clone written specifically for OS X Leopard and higher. This means that it has a native interface and tries to integrate with the operating system as good as possible. Examples of this are drag and drop support and QuickLook support.