Launchy - can't live without this one! more than a program launcher, it can launch internet queries at any site, it can run programs with command line switches, it can browse deep into your directory to open folders... run macros, scripts, batch files, control your media player... all without toughing your mouse
Billy - the perfect mp3 player. newest beta is great, just open an mp3 in your file browser, and Billy loads all the songs in the folder to a playlist. it saves the list when you exit though, so i load my whole song library, exit, then make the default playlist read-only. that way if i want to browse or shuffle, i just run Billy, but if i just want an album, i open an mp3. also scrobbles to last.fm, supports customizable global hotkeys, and has a tiny footprint. oh and the live search is amazing, no need to even bring up the window, just make sure Billy is focussed and start typing. super fast!
Xplorer2 Pro Portable -
not freeware unfortunately. the basic version is free but it's not portable. pro portable costs $40 but it's well worth the money. great file manager. best feature is the 'scrap container', which is like a virtual folder that you can fill with references to files. for example, send 10 different folders of pictures from across 3 drives into a scrap container, then multi-paste a desktop.ini file into them all at once to make them all display thumbnails. easy as pie. can also flatten entire trees into a scrap container, then filter/select/etc. i've used it for a year, and i still learn new tricks every day.
those are my top 3, can't-live-without programs. here are others i love:
q10 - a fullscreen text editor that keeps you focussed on the task at hand.
Shiruspad - a tiny note taking app that can pop up with a hotkey or a mouse hotspot. tabbed interface, url highlighting, and more.
IsUsedBy - drag a file/folder into the window to see what processes are keeping you from deleting it.
Real World Batch Picture Resizer - drag a photo onto the .exe to resize it. rename the .exe to determine the output size. also supports command line operations. very handy!
Aveicon - drag-n-drop to convert PNG<>ICO.
emu48 - an emulator for the HP 48 series RPN calculators. great stuff.