We're at a different stage of unschooling than you are with my three older kids in the 8-13 range and exceedingly self-sufficient on the internet, and my youngest happily going along for the ride wherever they're headed. So our focus has shifted away from "educational resources" that I ferret out. In other words, my kids use Google to look for information specifically related to problems they're attempting to solve or questions they're trying to answer, and I'm not really involved as an intermediary any more. Their research tends to be at a deeper level than is typically contained at 'educational' websites, so Google is their best friend :-).
I will put in a plug for Google Earth specifically. It has been a brilliant way for my kids to make the connection between the abstract world of maps and the concrete world of fields, cars and buildings.
Miranda