Not just a root kit, "Back Orifice 2000 is the most powerful network administration tool available for the Microsoft environment, bar none" (bo2k.com). It can be used to break a Windows system's (lack of) security (cars can be used to run over people too), but that isn't its main purpose; it's just a really good remote control package under the GNU General Public License.
Actually, BO2K is not that much different from other Windows remote control software such as CoSession Remote, Symantec's pcAnywhere, or Compaq's Carbon Copy, except pcAnywhere costs $169 and is not free software, whereas BO2K is under GNU GPL and takes eight times less RAM and 20 times less storage. A list of BO2K's features with respect to other remote control software can be found at http://bo2k.com/comparison.html
Back Orifice 2000. Show some control.