OS X does some periodic maintenance tasks at specific times. There are daily, weekly, and monthly periodic tasks that are launched in the middle of the night. If your Mac is not powered up 24 hours a day, probably some (or all) of those tasks are never (or seldom) performed.
You can find utilities around the net that allow you to manually launch those tasks, others (like anacron) that, once installed in the system, are repeatedly called to check any skipped cron task.
PseudoAnacron takes a different approach: at login (it's a startup application) it checks if the periodic daily/weekly/monthly task was skipped and in that case the task is launched; after that, PseudoAnacron quits, wasting no more CPU cycles.
- OS X 10.4 - 10.6.x. Universal Binary.