Web tools in the use of teaching

SinceShuchi's assignment remains but the link hasn't appeared yet, I thought I'll kick off the discussion here.
Starting by just listing web tools that can be used for teaching.

Email. This includes individual mail, group mail, replying, forwarding and replying to all options. Email also offers attachments facility so text, data and pics of a limited- now expanding since the arrival of Gmail - quantity of bytes can be sent along with it.

Chat messengers = Like Yahoo and MSN.
allows for real time conversations and enables group conversations too. with webcam and mics even video conferencing can be set up on chat messengers.
Chat rooms for specific topics can be set up.
Linking up with others through chat who are also interested in the topic of study is also sometimes possible in the common chat rooms.

Search engines : resource and info etc. data banks.

Blogs

Groups like the ones offered by MSN and Yahoo

Threaded discussion boards.

Radio and TV stations that can be set up by oneself - This is a very exciting option i want to explore.

Websites and webpages like the ones offered for free by Bravenet or Yahoo Geocities.

Software like Powerpoint is not mentioned because I do not consider tham webtools. They are computer tools.