Here are some useful links for students who are interested in developing their educational level. They offer some online lessons, tons of quizzes and many others.

ESL Quizzes Center (Very useful Language Exercises)

http://www.pacificnet.net/~sperling/quiz/

Internet TESL Journal links for students, with quizzes and crossword puzzles

http://www.aitech.ac.jp/~iteslj/s/

Daves ESL Cafe on the web

http://www.eslcafe.com 

Writing Den - part of the Digital Education Network

http://www2.actden.com/writ_den/index.htm

Online mysteries to read and solve

http://www.mysterynet.com

Knock-knock jokes and riddles

http://www.bayne.com/kaitlyn/

Learning oral English online

http://www.rong-chang.com/book/