Once a quarter, most NASA missions have to submit a chunk of archived data from their instruments to the Planetary Data System, or PDS. Those data submissions aren't due to show up until January 1, but two missions have come through a couple of weeks early with their data dumps. You can find the latest (and all previous) releases of Cassini data here -- the latest covers Cassini's activities from October 1 to December 31, 2007-- and the first 30 sols' worth of Phoenix data are here.
These data aren't in a format that is particularly friendly to the casual user; it's meant as a permanent archive, and it's kept in a format that's useful for scientists. The PDS has developed a fairly easy-to-use piece of software called NASAView (available for multiple platforms) that allows you to open images (you can download that here), and if you want to convert a big pile of data from the PDS format to an easier-to-use format, I wholeheartedly recommend Björn Jònsson's IMG2PNG.
The original JPEG images are good enough for most casual users who want to just browse through the images to see what a spacecraft's done. (In fact, even science teams make frequent use of the raw image pages for that purpose.) But if you want to process images -- make color versions, or mosaics -- it's worth your time to learn how to use the ones you can find on the PDS. Here's an example of how the calibrated images differ in subtle but important ways from the original raw JPEG versions. (Thanks to Peter Masek for pointing out this example.)