ESL Kid Stuff
http://www.eslkidstuff.com/OnlineGamesMain.htm#.VlITOWSrRpk
This site has a wealth of resources for ESL children, but it has a particularly great section of games and activities This site is appropriate for ELLs in elementary school.
ESL Kids World
http://eslkidsworld.com/Interactive%20games/Vocabulary%20Games/vocabularygames.html
This site is a collection of interactive games for children learning English as a Second Language. This site is appropriate for ELLs in elementary school.
Hello World: World Languages for Children
http://www.hello-world.com/languages.php/?language=English
This site has great activities, videos and games for beginning fluency level ELLs in elementary and middle school.
Dave's ESL Cafe
http://www.eslcafe.com/students/
ESL teacher Dave Sperling's in-depth site provides numerous lessons about sentence structure, word use, and more. This site is appropriate for ELLs in middle and high school.
ESL Student Lounge
http://www.esl-lounge.com/student/index.php
Hundreds of awesome exercises make this site a fantastic tool for students. Parts of speech and vocabulary are classified by difficulty. This site is appropriate for ELLs in middle and high school.
Activities for ESL Students
http://a4esl.org/
This website offers grammar quizzes at multiple degrees of difficulty and for learners from dozens of different language backgrounds. This site is appropriate for ELLs in middle and high school.