Blogging. For those of us who are - shall we say - unfamiliar, there's a whole world of it out there. There are currently around 100 million blogs out there (http://www.nybooks.com/articles/21013), with another 100 million now abandoned into the ether (including, I'm sure at least one that I started a couple of years ago!).
So obviously we're not going to go out there and read all of them, but it is possible to find a blog (or two) that have something to do with our interests.
And that's where teaching comes in. There are lots of educational themed blogs out there, but we can go a little further. Teachers are starting to use them with their students, with lots of opportunities available for online collaboration and discussion.
Check out these examples:
Mr. Sumner's page is pretty cool:
This one's Australian:
Food for thought.