GIMP
I'd like to see Portable GIMP; as well, I'd like to see the portable loader able to recognize that GTK+ is in ../lib/gtk+-2.0 instead of PortableAppname/lib/gtk+-2.0. This would allow for installation of Portable Gaim and Portable GIMP using the same GTK+.
GIMP would be awesome to show off to people. It really demonstrates a powerful software application that people can identify with.