Some very good portable apps not mentioned above (all of which are open source or freeware unless otherwise stated)-
AnyBurn (CD, DVD & Blu-ray burning app + a number of other functions)
Atlantis word processor (not free but inexpensive and excellent)
Bandizip (zip/archival utility)
CCleaner
Freac (audio ripping/conversion tool)
Jarte (word processor)
KeePass (password keeper and creator)
PDF-Exchange Viewer (pdf viewer with OCR plug-in)*
QtWeb browser (small lightweight web browser built in adblocker and download manager)
RevoUninstaller (uninstall utility)
- SumatraPDF mentioned by others above is both lightweight and excellent too.