The website I recommend today is for teachers of English as a Second or Foreign Language: BusyTeacher.org. It is full of worksheets that you can download to use in your lessons, and you can find activities for students of all levels. In order to download the worksheets you have to be registered in the site, but it is free. You can also upload your own materials to help other teachers and contribute to the education of hundreds of students from all over the world.
The interface is very easy to use. The worksheets are divided into categories, e.g. grammar, listening, pronunciation, reading, speaking, vocabulary, writing, teaching ideas, ESL articles, and seasonal activities. Each of these sections is also divided into sub-sections to make it easier for you to find what you are looking for. You can also browse the worksheets by writing your search in the searching box.
BusyTeacher is very useful for teachers because you can get ideas to create your own material, use some of the worksheets of this site in your class, or even adapt them to your own needs. If you are looking for something very specific, for example, an activity about Halloween for students of an intermediate level, you are very likely to find it.
I would say that it is a perfect complement for beginning teachers that still have not developed a lot of material of their own. I would like to thank all the teachers who share their materials with other people unselfishly! Thank you!