as machead3 said, you're seeing a kernel panic
sound like they are happening randomly for you? I'm guessing then it maybe a hardware issue, perhaps bad RAM (that's just one possible cause for a kernel panic)
when debugging a pesky Mac, I sometimes use a tool called Applejack - it requires installation prior to use - and then to run it you first need to boot in single user mode (hold Cmd-S while rebooting). Applejack will perform some basic maintenance tasks like running fsck on your disk, repairing permissions, etc.
however note: there is an optional memory test that you can install when you FIRST install Applejack. It is apparently a more thorough test than Apple Hardware Test. Before installing Applejack, please read the readme file and make sure you select the option to install memtest.
also: latest version of Applejack works on Mac OS X up to Leopard, but Snow Leopard is NOT yet supported.
another separate suggestion. Next time after you have a kernel panic and restart your Mac, open Console (in Applications/Utilities) and take a look through the log files. take a look at anything particular happening around the timestamp of when your machine crashed - sometimes there may be a clue of what's going on, especially if it's a repeatable cause.