Portable OpenOffice
John Haller has taken several of the best freeware applications and made portable versions. The outstanding OpenOffice works great from a USB drive and only requires 127 megs of space. You will find OpenOffice is well suited to large USB drives like the SanDisk Cruzer. http://johnhaller.com/jh/useful_stuff/portable_openoffice

Free Portable Word Processor
Portable AbiWord

If you don't want or need a full office suite, take a look at Portable AbiWord. It's much smaller than OpenOffice and faster, too. John Haller created this version of the excellent AbiWord word processor to perform even faster than the original.
For additional goodness, as of version 2.4, AbiWord includes a grammar checker.
http://johnhaller.com/jh/useful_stuff/portable_abiword/

Free Portable PDF Reader
Foxit PDFReader
Weighing in at a little over a meg, this PDF viewer is faster than Adobe Reader. Believe it or not, you might come across a machine that doesn't have Adobe Reader installed. You might even prefer Foxit. A lot of people do.
http://www.foxitsoftware.com/

Free Portable Calendar / Personal Information Manager (PIM)
EssentialPIM
It performs all your basic PIM needs: calendar, contact manager, etc. Conveniently, you can import your Outlook information, and it supports built-in encryption. You don't want your whole life in someone's hands when you lose your keys, do you?
http://www.essentialpim.com/

Free Portable Note Taker / Information Manager
VectorNotes
A simple way of keeping simple text notes throughout the simple day, then take them with you when you go. Jot down that name, shopping list item, or perpetual motion machine idea. Find it later by doing a speedy search.
http://webpages.charter.net/bobad/vectornotes.htm

Free Portable Text Editor / Notepad Replacement
Notepad2
This is a lean, mean text-editing machine. Notepad2 is one of the lighter-weight of the notepad replacements, but it's still vastly superior to Windows Notepad.

Free Portable File Undelete Tool
Restoration

If you are truly paranoid about leaving tracks on a visited computer, run Restoration to see what files may be undeleted after you leave.
http://www.snapfiles.com/get/restoration.html