Huzuni is a very cool hacked client made for griefing in Minecraft. It has lots of good features with different options, you can customize the xray, nuker, flight speed etc. from within the GUI or via commands. It also has its own Huzuni options menu, from here you can chose another font, how the GUI style should be and various other options. Overall a pretty good client, also for the unexperienced.
Right Shift = GUI
.help = help command