Anki

http://www.ichi2.net/anki/

Multi-platform, including iPhone and Droid, free. Auto-sync cross multiple computers. Uses SuperMemo memorization algorithms.

Mnemosyne Project

http://www.mnemosyne-proj.org/

Flashcard-based, spaced repetition, multi-platform, uses PyQt.

History:
SuperMemo (Windows-only, commercial, Piotr Wozniak, 1987) >> MemAid (multi-platform, commercial, David Calinski, 2003) >> Mnemosyne Project (open-source)

Super Memo

http://www.supermemo.com/

The "father" of all Flashcard tools.

Works by "clipping" a piece of text or a picture. Windows-based, $60. Feels dated.

Quizlet and Lumosity

Use Quizlet for flashcard-style memorization (they "crowdsource" the flashcard sets and they have lots of them).

Use Lumosity games for general improvement of attention, memory, etc.