A current implementation is
sleepless
However: 10.3 uses a new power management thingy, which turns the computer off if it has it's lid closed and hasn't gone to sleep for 10 minutes, making the software much less useful under 10.3
Another note, the kext used in this is called insomnia, i've made a couple applescripts to load and unload it, rather than waisting more menubar space.
--will