Sheppard Software is something I recommend to pretty much everybody. There’s no actual software to download, you simply visit their site. There aren’t too many websites that equal what they have to offer for free, and the topics are good for a wide age range.
Specifically, I appreciate the geography section–my son has used it for years and he took 3rd place in the National Geographic State Bee this year! I can’t say that it was solely due to this site (not at all) but Sheppard really gave him a great start. It starts out easy and introduces the different countries or states, and gets progressively harder until you have to recall the names from memory.
Sheppard Software also has sections about animals, chemistry and others including special sections just for preschoolers. The only negative that’s been brought to my attention is the ads, which I’ve never noticed because we have adblock on our browser.
If you’d like to check out their website, head over here.