Very good lesson ideas for Elementary school and Junior high school. Covers the textbooks for Junior high, as well as non-textbook lessons. Very useful site.
Christmas lesson ideas
Phonetics and how to teach them to Japanese students
Worksheets
Great website for JHS ALTs

http://www.mes-english.com/
MES English – Printable flashcards, games, worksheets etc. Really excellent site.

http://www.the-bus-stop.net/free.html
The Bus Stop Go to the free resources section for lesson ideas, flashcards and games. Some printable lessons.

http://www.puzzlechoice.com/pc/Kids_Choicex.html
Puzzle choice – Printable puzzles

http://iteslj.org/games/
ITESLJ – Games and activities for ESL classes

http://www.esl-galaxy.com/
ESL Galaxy – Printable worksheets, games, lesson ideas etc

http://www.englishbanana.com/
English Banana - Games, worksheets and workbooks

http://www.english-4kids.com/
English 4 kids – Worksheets, games, and guides for using video (incl. Mr Bean and Harry Potter) in the classroom

http://www.teachenglishinasia.net/tefl-tesl-games-and-activities-directory
Teach English in Asia - Directory of games and activities sorted by level and target activity

http://www.teachers-pet.org/
Teachers pet - Toolbar that can make crosswords, bingo cards, jumbles, word search puzzles, flashcards, cloze tests, pair-matching puzzles etc.

http://bogglesworldesl.com/
Lanternfish – Worksheets, games, flashcards etc

http://puzzlemaker.discoveryeducation.com/
Discovery Education Puzzlemaker – Puzzlemaker site and some lesson plans

http://www.eslteachersboard.com/
ESL Teachers Board – Lesson plans, stories, printable handouts

http://www.rong-chang.com/qa2/index.html
Short stories – 100 short stories to use with classes. Some are pretty random but some would be good.

http://www.teachenglish.ws/
Teach English - Game ideas and a Bingo card generator

http://genkienglish.net/
General website with lots of ideas. Most ALTs use this website

http://eslgames.com/
A nice site for teaching music to Japanese students.
Games, games, games!
This site is good for older students. It breaks down nuances with grammar and is focussed more on communication

Japanese website with nice picture cards.
Good site for Eigo-Noto and lesson planning with your Japanese HRT