Portable OpenOffice
Yet another example of how distributed development (the open source model) can produce superior software innovation, is PortableApps.com. They have customized various FOSS applications to be capable of running from a flash disk, without installation on the host computer.
John T. Haller is the force behind this project, so check out his site too.
Among others, Firefox, GAIM, and AbiWord are all represented, but, Portable OpenOffice is perhaps my favorite.