I realize I've been referring to these and haven't introduced them. We do use a lot of web based resources, some free, some not.
Enchanted Learning: One of my favorites! Worksheets, crafts, projects and web based fun educational stuff for preschool through older elementary. Great for themes. You could easily home school just from this site for preschool through 3rd or 4th grade. Some stuff is free, but it is worth the $20 a year to have access to the easier printing/no ads/better sizes.
Learning Pages: This is free but you have to sign up (and they've never sent me spam). The ultimate in themes for Pre K through grade 3, arranged by topic, grade, and class (language arts, math and science). Extras include fact files, lesson plans, murals and cut outs. They also offer printable books, monthly themed worksheets and more. I think they actually have other stuff if you pay them money, but I've never tried it.
Brain Pop: Great funny highly educational movies on any topic you can imagine with other educational bonuses (quizzes, work sheets, and activities). Very expensive! I think it's something like $75 a year, possibly more, but you can get a 14 day free trial to see if it is worth it for you. The one caveat I have for this site is that I can't let my younger kids loose on it. Some of the topics (like reproduction) are topics I want to introduce to them, some are just aimed at older kids (like AIDS), and some of the novels (like Frankenstein) are books I want them to read before they see a discussion of them. I also have found their discussion of controversial topics to be fairly left leaning, their movie on stem cells was disingenuous at best (some people object to using embryonic stem cells because they could become embryos?). Still worth it to me with supervision.