CoCoRahs – A collaborative effort to measure rain, hail and snow across the country. They have training slideshows, and send a free rain gauge and hail pad.
http://www.cocorahs.org/

Projects by Jen - Site created by teacher Jennifer Wagner, offers simple projects classrooms can join and participate in with other classrooms across the US.
http://www.projectsbyjen.com/

Journey North - Initially, this site tracked the passage of Monarch butterflies, but now offers a number of projects based on the passing of seasons.
http://www.learner.org/jnorth/

Journey North for Kids - A version for younger students, with resources on seasonal change.
http://www.learner.org/jnorth/KidsJourneyNorth.html

K-3 Winter Wonderland - Winter themed technology projects for K-3.
http://winterwonderland.wikispaces.com/

How Tall is a First Grader - Project connected to math, reading and writing skills, and measuring activities. Teachers measure first graders at the beginning of the school year then at the end of the school year. Results are posted, and you can compare your results with results from all over the country.
http://howtall.wikispaces.com/

The Monster Project - Partner schools take turns writing a written description of a "monster" then drawing a picture of the "monster" based on the partner school's description.
http://monsterproject.wikispaces.com/Welcome

Time Zone Project - wiki designed to help students learn about time zones, and what children in other time zones are doing throughout their day.
http://timezoneexperiences.wikispaces.com/For+Teachers

epals
Under the tab marked "collaborate" – search by classroom profile, or by country to find a partner classroom to set up as an epal, or do some sort of project together.
http://www.epals.com/

Skype Authors - Provides a list of authors who will participate in a 15 minute, free, Skype interview. Arrangements can be made to have a longer Skype interview, for a fee.
http://skypeanauthor.wetpaint.com/

50 Awesome ways to use Skype in your classroom - Article has links to 50 ways you can use Skype in your classroom. Some pretty intriguing ideas.
http://www.teachingdegree.org/2009/06/30/50-awesome-ways-to-use-skype-in-the-classroom/

iEARN Projects – Offers projects your students can participate in with global partners. Use the "Find a Project" window to set parameters to view projects that are appropriate to the age of children you teach.
http://media.iearn.org/projects

K-12 Collaborative Science Projects
Collaborative projects, arranged by grade level.
http://www.k12science.org/currichome.html

KidLink - easiest way to describe this is collaborative projects using a blog type setting.
http://www.kidlink.org/kidspace/start.php?HoldNode=614