One forum that I recommend to my colleagues is ProTeacher Many teachers have gotten creative and refreshing new ideas, that it make this site a win-win for all that review it daily.
Discussion boards by grade level, allowing you to communicate and collaborate with teachers around the globe who teach the same age students as you. Perfect for brainstorming your new lesson ideas.
Discussion boards by topic, such as Guided Reading and Literature, Science and classroom management, allowing you to share and collaborate with other teachers who teach the same topics, no matter the age of the student.
Live chat, allowing you to sit and have a conversation with other educators beyond the walls of your school, thus expanding your knowledge base on the topic.
Collections of teaching ideas and lesson plans on hundreds of topics, with information contributed by teachers around all over the world.
Blogs, and lots of them, with thoughts from specific teachers, and sometimes comments from readers.