I’ve recommended Twitter already, in Tool #53, but here is a long list of ideas about how to use twitter for learning and teaching.
Twitter is very, very popular these days. It is gaining in popularity in Japan. You post short messages about what you are doing thinking or discovering, and people follow you, they can read these posts. It is easy to set up a class like this and get feedback on your lectures, but there are many more ways to use this tool.