I found a very cool Mars-related web site the other day, Exploremarsnow.org, but unfortunately it seems to have been abandoned in 2003. I didn't notice this until I had looked around for a while, found a page for signing up as a member (promising access to additional content), and paid for a one year membership ($15) with PayPal. Oops. Read the fine print before signing up for something. Oh well.
It's still a cool if limited site to explore. You can't really explore Mars, but you can explore a detailed and pretty realistic interactive model of a Mars Direct-like "hab" that has landed on Mars, and the graphics and the interactive "walk through" of the various parts of the spacecraft are quite nicely done (red highlights indicate transitions/doors). There are also many mouse-active "hot spots" (yellow highlights) that display pop-up text boxes explaining the selected object. NOTE: Interactive features would only work for me in Internet Explorer, not in Firefox.
This site appears to have won some awards back in 2002-2003 and it really is a great start on an interactive Mars exploration site. It sounds like they just ran out of development funds, though someone is obviously still hosting it. They should leave it up but deactivate the membership stuff. So go have a look around, but don't get carried away and pay to sign up like I did!