Time Warp
I often have to resorting to resetting the system clock for debugging, testing, or avoiding stupid restrictions in software. Doing so is messy, since it screws with everything else on the system. After having to do it a lot yesterday, I wrote Time Warp, which is a tiny OS X dynamic library (plus two hackish front-end scripts) that intercepts system calls to gettimeofday (upon which all of the other time-related functions are implemented).
Sample use looks something like:
./time-warp -2M cal
Which outputs a calendar for 2 months ago, or:
./app -14d /Applications/iCal.app
which runs iCal thinking that it’s two weeks ago. You can download it here. Cue joke about using it to save time.