Some that use (trying very hard to migrate to near total open source)
PC:
VLC (standard def video)
Media Player classic (HD Video though VLC is catching up with hardware decoding)
WinAMP (music playback, this one is kind of a throwback but it remains the most unobtrusive music play with the right skin and widow size IMO)
GIMP (image editing)
Boxee (home media center. I Might change though)
Handbreak (video conversion has recently replaced DVD Decrypter and/or DVD Shrink)
uTorrent
Minefield! (New Firefox alfa containing Tab Candy the most awesomest revelation in recent browser history)
7zip (best compression utility evar!)
graphCalc (graphing calculator replacement for windows calc)
open office (gets better every update)
Ditto (tracks and stores ALL items saved to your clipboard!)
Steam (if you have to ask you'll never know!)
Virtual Clone Drive (Disc mounting)
Spybot SD, CWShredder, Hijack this (for when I get teh AIDS)
DBaN (for total data destruction!)
BackTrack 3 (need to upgrade but I'm still learning ....

)
PuTTY (Telnet and SSH for Windows and Unix platforms)
Microsoft network monitor 3.4
Skype
Mac:
VLC
Boxee
Burn (slightly more detailed extensions for OSX's native burning utility)
uTorrent
carbon copy cloner (Used occasionally for data backup)
WaterRoof (IPFW Firewall Frontend Still debating NoobProof)
Minefield!
Drop Folders (automating converting of video files with Handbrake)
Handbrake
Stuffit expander (compression utility, requires a junk email address to register for product ads)
Tinkertool (OSX hidden settings exposer)
Open Office
Steam
PuTTY
Skype
Experimenting with Squared 5 (MPEG Streamclip is a powerful high-quality video converter, player, editor for MPEG, QuickTime, transport streams, iPod. And now it is a DivX editor and encoding machine, and even a stream and YouTube downloader.)
These are just what I can think of off the top of my head there's more! Anytime I feel lonely I head over to SourceForge and look for new stuff (though not out here). I'm still searching for a decent burning ROM for windows and Open Office still has a long way to come with MS Office integration but it is by far the best suite available.