It's fun to check things off of a to-do list, but often what's more important is remembering when you last did something. Is it time to change the water filter? Get a new toothbrush? Go to the doctor? If you had something in your calendar the last time you did it, it's fairly easy to check how long its been, but if not you're out of luck.
Recur! solves that problem by keeping track of how long its been since you did the tasks you enter. You can use it for recording achievements, or for checking the last time you cleaned the kitty litter. You can set reminders so once a certain amount of time has elapsed you're prompted to do the task again. Recur!, which is £0.99 on iTunes, is made by the same developer that created CARROT, but it's more about being straightforward and simple than about bringing the snark. Oh well. [Recur!]