Typing Web
Free with registration. Multiple levels and certificates
Typing Tutor online
http://www.typingtutor-online.com/EN/Aspx/SelectExerciseWithCharacters.aspx
Another free site with multi-levels
Sense Lang
http://www.sense-lang.org/typing/tutor/keyboarding.php
Another free site with multi-levels
Type Racer
You can practice typing skills or race against others online.