I see 1st - 5th grade. For my 3rd-5th graders I use a blog for vacation assignments tied to curriculum areas.
I see 600 students, all 1x a week, so I try to extend what we do over the vacation weeks.
I've also used the blog for in class assignments.
I also have a wiki where my 5th graders are starting to post research-based Keynote presentations once they are completed.
(it's a work-in-progress)
In a general way I use Diigo to bookmark and share useful sites. Not used specifically in-class, but available to students as a resource.
One of my favorites and most used Web 2.0 Resources is the Educational version of Glogster.
The best way to sum up Glogster is to think of it as a Digital Scrapbooking. If you haven't checked it out, I blogged about it here. I also inc...
Make sure you are on the Educational version of Glogster and not the regular version. The regular version may not be appropriate for school.
We use it to create All About Me Glogs at the beginning of the year, we use them to create glogs about Famous People we research. We have used Glogster to create Internet Safety Posters, etc. Lots of different ways you can use it.
If I had to name 1 resource that I use a lot and love, it would be Glogster.