Flashcards, picture dictionaries, games, writing worksheets, listening worksheets, crosswords, word searches and more! I use this site for lower levels. You can use the included materials/vocabulary or create your own with toolsforeducators.com, a sister site that uses MES-English images and vocab.
Online activities that use pictures and animations to teach vocabulary. Though the site is a bit busy and dated, it is quite useful for beginning students, and there's even a pretty comprehensive Citizenship section.
It's a little difficult to get around, so look for ESL for Adults, Business English, and Travel English for adult-friendly dialogues. The games section is also full of board games that match many grammatical concepts. Low-Level SUB LESSONS under "Travel English."
Quizlet is a site where you can create vocabulary lists for your classes or search for ready-made lists (many include audio as well!). Students can then practice with flash cards or games. (You can find my sets here).