SmartCards+ - Spaced Repetition Software for iOS. Modern, powerful and easy to use.
Reverse Dictionary - Describe a concept and get back a list of words
FlipToMind - Space repetition app to help you remember everything
Meaning Monkey - Find the definition and meaning of anything.
Zorbi - Spaced-Repetition Flashcards - Other tools are built by boomers, Zorbi is built by students.
Silicon Valley Dictionary - Urban dictionary meets Silicon Valley