www.englishpage.com (excellent grammar exercises)

http://www.perfect-english-grammar.com/index.html

www.englishlearner.com/tests/index.shtml

http://www.englishclub.com/learn-english.htm

www.nonstopenglish.com/

www.better-english.com/vocabulary.htm (esp. business vocabulary)

www.usingenglish.com/students.html (idioms & phrasal verbs)