It is bit hard to make short description for PortableApps since it's both web-site, software itself and platform for other software as well.
Basically their site offers suite (launcher for external media like flash drives) and number of applications tailored to be portable and work with suite (or without it). Selection of apps grew to be pretty impressive and includes both well known titles like OpenOffice and Firefox to homebrew like Toucan (that won't ever be well known unless made bit more useful and sane :) ).
I don't use their suite (I have self-coded launcher for my needs) and some apps presented are portable natively, but it is still good place to look for portable conversions of well-known stuff (some of which I do use).
Most importantly PortableApps simplifies process a lot for casual users by removing need to tune and convert software into portable mode (even if that is its’ native capability) and offering consistent experience.
Home page http://portableapps.com/
Download page http://portableapps.com/suite