Kind of fond of FaunOS
FaunOS offers a full KDE desktop system with a comprehensive set of applications as either a live DVD or a live USB flash drive. The USB format is the distro's primary focus. FaunOS is based upon Arch Linux, and ships with Arch's package management system. The more I tested FaunOS, the more impressed I was.
USB memory disks have many advantages over CD or DVD disks. They require no separate drive with movable parts, expose no data areas to become damaged, provide faster operation, and fit in a smaller space. FaunOS offers a current downloadable image at no charge for use on ordinary USB memory sticks. The distribution requires a USB stick with a minimum of 1GB capacity, as the image is 934MB. I recommend using a larger capacity device if you'd like to save your customizations.
In addition to the USB drive itself, FaunOS requires an x86 machine with at least a 500MHz CPU and 512MB RAM. If your target machine isn't bootable from a USB device, you can make a special bootstrapping CD to boot from.
The instructions for installing FaunOS to USB media are simple.