Here are some of my favorite resources for finding great stuff about teaching English on the internet.
http://www.teachers-pet.org Awesome Word toolbar to make quick work of making crosswords, bingo sheets, and loads more!
Bogglesworldesl.com Fantastic collection of free resources covering everything under the sun. Lots of updates and seasonal activities.
MES English Another great site I use all the time for really high quality and engaging flash cards and games. Well worth checking out.
Discovery School Puzzlemaker Saves my skin on a daily basis. Make up some entertaining puzzles for your class.
ESL Teachers Board Free teaching materials, jobs and resumes.
Useful real world resources for learners
I often use the Guardian and Mainichi daily news online newspapers as a source of material for advanced learners.
Another resource which is fun is youtube which is great for grabbing commercials, snippets of TV programs from around the world etc. Good if you have a PC in the class.
A useful bunch of short stories can be used for reading, vocab and to help with writing style.