Cygwin would be an excellent candidate for inclusion as a true "Portable App".
It's well suited because of the plethora of utilities that are available with a Cygwin install.
Just the 'rsync' utility alone is enough to make it worthwhile.
A portable rsync would make it extremely easy to create backups of data without going through the pain of doing a full cygwin install on the local machine.
(ailing Windoze machine... I'm sure we've all had to deal w/ one of those)
Essentially Portable Cygwin =
I've actually found some instructions on installing Cygwin in a portable-friendly manner. In fact, I was able to get it to function (somewhat) off of a USB flash drive.
However, it surely don't "just work". I had to go through a few hoops to set it up and there are still a number of oddities that prevent full usage.
Damn, I guess I'm also seconding this thread: