Schoology is a free service that allows you to create a social network-type environment for online instruction. Think of it as a combination of Moodle and Facebook and you've got a pretty good idea of what it is like.
To get more into the service, you need to create an account. If you don't want to (or can't for some reason) create an account, but are interested in the service, let me know and I will get you access.
Smartkiddies is a completely free math practice site out of Australia for students in K-6. After creating a teacher account, you can set up a class with usernames and passwords for up to 75 students per class. After creating a class, you can assign instructional videos, activities, worksheets, assessments, and enrichment opportunities. Progress reports and messaging are built in to help you monitor and communicate about student progress. Another great feature is the ability to practice basic math facts and concepts by competing against other kids from around the world.
This "game" allows kids to participate in a science mission lead by scientists. They investigate if and why there is an increase in the shark population in an area. Information is incorporated in the game using online resources and video. They actually have to count the sharks in the area- so they have to go through stages in the scientific method. I think students will love this.