A very extensive wiki with links to a huge number of Web2.0 tools, such as presentation programs, mindmapping, comics, image sharing and editing, search, etc.
Mailvu is a free video messaging system that looks simple and easy to use. R. Stannard's video helps you get started quickly, and he also discusses how he uses the tools with his students for language learning: you can have students send you video recordings and then comment on them, have students do interviews of each other, describe something, talk about a holiday or their weekend, etc. Mailvu also has apps for Android and iPhone.
Very short videos--36 sec--may be good for low level students to practice initial speaking/listening. Also has apps for various smart phones. Might be good for a quite pronunciation quiz, for example. The teacher can then send back a short video message with corrections, modeling, etc.
A video on how to use Keek to make and share short video messages. Although limited to only 36 secs, this might be a good application for beginning learners, or for a quick pronunciation quiz (you will see who is taking that quiz!) You can also embed a finished recording in your blog or wiki, as well as sending it by email. A good way to have students create a short, practiced conversation. Also has smartphone apps for mobile recordings, RSS feed to follow, and links to Facebook and Twitter.
This might be a great tool for an extended project, e.g., have your students create an infospot audio guide to their local community. Lots of examples are linked on the front page, and there is an iPhone app to scan, listen, and record wherever you happen to be. There are currently over 1300 guides created by users, and more coming.
This is a great digital images project using photos of graffiti on Flickr, and a spreadsheet of questions on Google Forms. Students sign in to select images that will support the ideas they write about. Thanks to Webhead Sasa Sirk, Carla Areana, Bee Dieu, and Rita Zeinstejer!! Also included are links to posts that Sasa's students made and responses from Rita Z's student sin Argentina. Nice example of collaborative Web 2.0. (Caution, some graffiti have adult language and themes!) See also http://wallstalking.org/.
Teaching social bookmarking is the topic of this K-12 blog entry. Tells how and why to use Diigo educationally, but can be applied to other, similar applications.