Preferably one with no registration required (IE no runescape)
http://chibitami.net/works/flash/cirno_training/index.html
A short game. Pick the answer that is incorrect.(unless its 1 + 8, or anything else that actually adds up to 9. Then you pick 9)
http://www.coolmath-games.com/0-bloons-tower-defense-3/index.html
An easy tower defense-styled game.