- Weather, ecology and energy are presented to kids from about 10 to 15 in really cool multimedia units. (Bright, interested younger kids might enjoy these also.) There are videos, activities and printables all available for free. It's easy to teach, or your self-starter can do it on her own. Not enough for a year of science, but certainly good for some variety. Has my kids' Great rating.
- As a confirmed Linux user (can't wait for Ubuntu Karmic Koala!!!), I've been hunting Open Source educational materials as well. Curriki is the home of thousands of such projects - some ongoing, some finished. There's even a homeschool section. This is the home of the software developed for the One Laptop per Child initiative, which means that much of the curriculum is web-based rather than text-based. It covers from toddlerhood to college. Submissions are welcome also. Every time I go there, I'm amazed!
Teachers Pay Teachers
Teachers (including homeschool types) put their best lessons, power point presentations, worksheets etc online, and other teachers (again including us radicals) throw them a few bucks to get the resources. Again, it's little ones on up, and there are quite a few few freebies. The previews are good, also. It's possible to make a bit of money on this site. The indexing is a bit clunky, though.
If you don't have Power Point and would like to use the ones offered on any of the sites above, try Open Office, a free, open souce program by Sun Microsystems that is much better than MS Office and includes quite a bit that MS doesn't. And it's awesome.
There are quite a few more to add to this list...but this should offer a nice beginning!