It"s completely up to you. If you want basic exercises, livemocha is good & if you are Advanced or Upper-Intermediate, Englishbaby or Ebaby is appropriate for you but it"s hard for lower levels. Just take a look at them, I"ll see later if I can find some other good ones. http://www.usingenglish.com/members/tests/
http://www.usingenglish.com/quizzes/
I must check the others because I"ve forgotten their names.