Here's my list. I know, I was only supposed to submit 10 - Sorry; there's just too many honorable mentions
Collectively, all my portable apps are under 200 meg of thumb-drive storage. I use all of my portable software on all my notebook and desktop PCs.
1. Free Commander - In my humble opinion, it's simply the best free File Manager
2. Firefox - Using PortableApps.com version, but I have it tweaked to the max. Still waiting for K-Meleon to catch up
3. 7-Zip - Compress/Decompress everything I use
4. NotePad2 - Very small and fast. This is a great text editor.
5. CCleaner - Still my favorite cache cleaner
6. Photo Filtre - Small, fast, easy to edit graphics files
7. RegScanner - Easily search the registry. No more F3 in RegEdit
8. DeFraggler - My pick to defragment any hard drive
9. MPlayer - plays all of my video files, small & compact
10. 1by1 - Can't beat the size - plays all my music from my folders while I work
Also, Toolbox is my portable app launcher. Its easily customized to my taste with all of my app icons in one place.
Didn't make the Top 10, but noted as frequently used tools...
SoftMaker Office 2006 - TextMaker & PlanMaker. NO MS Office here!
TextMaker Viewer 2008 - I use this every day. Thanks to JauntePE and joby_toss
Free Windows Registry Repair - Great registry cleaning
System Explorer - No Task Manager here - I can see everything running, easy to customize and use
File and Folder Unlocker - Unlock locked files using drag and drop
SysProt AntiRootKit - Free and portable, great tool for quick cleanup. Easily kill those rootkit hooks.
Simple Machine Protect - Portable, thanks to JauntePE
Neutron - Easy way to sync time without using another bloated Windows Service
ADRC Data Recovery - Single executable. Free and portable file recovery