Polling and Student Response
This is really great for implementing BYOD. Kids love to bust out their cell phones. It's a great tool for discussions, feedback, quizzes, and surveys
Any type of quick assessment that you might require from your students can be sent as a Google Form, which automatically sends you the data from the results as they come. This is also good for having your students create a list for you by sending you their information in a Google Form.
Cel.ly is designed to look more like a social networking site, also great for implementing BYOD. Kids would really get into this one because it feels like they are hanging out with friends while still learning.
Every survey I've ever had to take for work has been through Survey Monkey. Good for formative assessment. There is a free option, and a Pro option.