The U3 platform came on the Sandisk Cruzer Micro 4GB flash drive I recently purchased and instead of formatting it into oblivion I actually used it.  Apps I already used in a portable fashion (on the Cruzer Micro 512MB it’s complementing) like Firefox, PuTTY, and FileZilla have U3 builds, as well as some other ones I haven’t previously used portably like WinSCP and OpenOffice.  It’s a pretty slick platform, although I have to think Sandisk was inspired by John Haller‘s PortableApps.com work.  Looks like he published a similar platform that I hadn’t previously seen or used.

U3 on Wikipedia