Exploring Mars and measuring craters
For those wishing to explore Mars, you still can do it with Skyline TerraExplorer. A free Mars virtual globe can be found on their main website. Look for the Mars Rover Mission thumbnail on the left side of the page; when you click it, further instructions for loading their globes browser will appear. Since TerraExplorer possesses several measuring tools, including distance, slope, vertical distance, and area, I had fun measuring the area and depths of craters near the Spirit landing site.
Windows-based NASA World Wind lets you explore Mars too, but it requires a plug-in. Actually, two Mars plug-in versions exist: a low-resolution (18MB) and a high-resolution (3GB). I've only installed the low-resolution version, but it was simple. It self-installed, and when NASA World Wind started, the option for Mars existed under the main File menu. The high-resolution version requires a little more installation expertise--including the need to load about 3GB of imagery onto your computer. The good news is that once it's loaded, you do not need an internet connection to explore Mars with fairly high-resolution imagery. For details on these plug-ins for NASA World Wind, visit the World Wind Wiki.
Several non-georeferenced Quicktime VR Mars panoramas can be found at www.panoramas.dk.