There are many sites that are set up particularly to support the integration of the Internet into the classroom in hands-on collaborative ways. Below are a few examples. This is certainly not comprehensive. For a fairly complete listing of interactive projects on-line you should check in at the Global SchoolNet Internet Projects Registry. This is a good general bookmark actually because it allows teachers to advertise their own projects to find partner classrooms for collaboration. Also if you have experience with using these sorts of projects, I'd love to here about your experience. 

Here is a special site which is particularly timely (October), Bats Thematic Unit

The Monster Exchange Project
K-8 students try to communicate an original monster image into another child's mind using learned writing skills and technology.

The UN's CyberSchool Bus

Eyes on Art

Thinkquest
This site encourages team-building and on-line constructivist learning by awarding grants to students (12-19) for the development of websites, such as Stamp on Black History, to foster the learning of other students. Great site!

KIDLINK
Global Networking for Young 10-15

CNN's Interactive Learning Resource

Cranes for Peace
This page is used with the book Sadako and the Thousand Cranes

WorldLinks for Development program

Turner Learning Network (although this costs money)

Education First's Black History Archives

The real scoop on Tobacco

And for a bunch more, see Global Schoolhouse's list of Projects and Programs

Blue Web'N is another good overall index of sites with rating and descriptions. It's also organized by subject area.