The recent popularity of Mars in the news has driven a lot of traffic to our Geology of Mars Web site. Where you can learn about the six geological processes that are either currently operating on Mars or have operated during Martian history.
Though the site was completed in 2001, the information is still very relevant. When Mars made its closest appearance to our planet in nearly 60,000 years, the Geology of Mars site was chosen as a Yahoo Pick and an Eisenhower National Clearinghouse favorite among others. Now the Spirit and Opportunity missions to Mars have rekindled the popularity of site again.
And while on the topic of Mars: you can download a scaled-down version of the program that NASA scientists use to operate Spirit and Opportunity. I can’t say the interface is great... it’s a mix of sequential narratives and an image browser. Take a look.