In the last couple of weeks, I’ve discovered two projects which both boot from CD, don’t install anything on the PC, and provide interesting functionality.

The first of these ix GeeXbox, which you can get from theGeeXboX HomePage
When booted, GeeXbox turns your PC into a dedicated media player. It boots a version of the Linux kernel, and then uses the popular mplayer to allow you to play DVDs, VCDs, or regular audio CDs. It also allows you to play pretty much any multimedia file from your hard drive, and is supposed to mount Samba shares as well (as yet untried). Best of all, the ISO is a mere 4.3 megabytes, so it is quick to download and burn.
Nifty!

The second of this is KNOPPIX, which is a bootable version of Linux. It runs Linux live off the CDROM, and includes mp3 players, web browsers and all sorts of goodies. I keep one around as a repair disk, since it can mount most filesystems and autodetects networking setups.
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Both are highly recommended.