Totalus is essential: We all have to calculate something everyday, e.g. planning of shopping, summing up of receipts, counting of calories, etc. Most of such a calculation just has a simple form of summation of multiplication results.
Totalus was designed just for this type of calculation, where you can get a total of numbers only by typing them, you can leave annotation for each number and you can try out various plan by temporally excluding some of them from the calculation. Besides, Totalus automatically saves and loads windows like Stickies.
These features will make Totalus one of the most convenient tools on your Mac.