If you have to/ want to remember a lot of stuff...
Brilliant app. Lots of info online about the scientific study behind the algorithm spaced repetitions.
Simple to create your own courses and sync them with supermemo.net. Plenty of info for this too. Some decent shared courses available.
I have tried some of the paid for vocabulary for French, which are good. NB they are actually rented for three months not bought.
Look out for specials on the app store because the full price would be really stretching the friendship.
nonsequitur73 about
SuperMemo (old), v2.3