All systems: If you're keen on trying out one of those clever devices from the One Laptop Per Child project, you can get halfway there by running the OLPC's operating system from a USB drive.
Sugar on a Stick, a thumb-drive-friendly re-build of the Linux-based Sugar OS found on the XO laptops, can be loaded onto a a portable drive from a Windows, Mac, or Linux system. It's basically creating a portable Fedora system with the OLPC interface layered on top (and, we'd presume, most of Fedora's own tools stripped out). It's great for OS voyeurs, or for letting your kids try out the education-focused software behind the OLPC project.
The Sugar Labs wiki has all the files and instructions you need to get started with your own portable Sugar setup. If you want to skip the thumb drive boot and try out Sugar in a virtual machine, there's a pre-made virtual disk image you can load into VMWare or VirtualBox (and here's our guide to getting started with virtual machines to help you out).
Sugar on a Stick is a free download for Windows, Mac, or Linux systems.