Question Tools is a program with a server client that allows the generation of interactive testing. It takes a certain amount of technical knowledge to set up the server, but test generation is easy.
Zoomerang is an online polling & surveying site. The basic, aka free, account allows you to create an unlimited number of surveys & polls, each having up to 12 questions, and 100 responses. Results can be viewed in real-time. This is ideal for surveying small population like a class or grade level in a school, or even as a virtual testing/quizzing tool!
I would use Tumblr in my classroom as a platform where my posts would be the primary source and students would be requested to add a post to my initial post after every class with any questions, comments or concerns they had about the lesson. This would be an integral part of the classroom which would be introduced on the first day of class and would be followed throughout the duration of the classroom. Responses to the posts would be factored in to the student's final grade.
This site allows for students to collaborate with other on a website. Students can ask ask other questions and answer those, allow who you want and it has great setting for students.