An older, highly stable exploit for early Xbox 360 models that allows for near-instant boot times.
Viva Piñata: Party Animals on JTAG/RGH — The Ultimate Setup Guide Viva Pinata Party Animals -Jtag RGH-
A more modern and widely compatible modification that uses a "glitch" chip to bypass security. While boot times can vary (from 2 seconds to 50+ seconds), it works on almost all Xbox 360 revisions. An older, highly stable exploit for early Xbox