Not sure if all are OSS, but are free.
Firefox for the internet. (with adblock plus and flashblock)
Handbrake to rip dvds to video files to my server (and sync up with a rifftrax)
Media Browser to play the videos on my media center PC
Shark007s codec pack to setup haali's splitter and configure around Windows media foundation crap.
Media Player Classic Home Cinema (used as external media player for mp4, ogm, and mkv files)
MKVToolnix to mux synced up rifftrax audio with the movie file into an mkv.
LineIn to route audio inputs to an output (so I can run all the audio from my 360 or gaming PC into my laptop so I can listen to music or podcasts at the same time, or whatever)
Soundflower (and soundflowerbed) to route audio from one application to another
Would use MapTools if I played RPGs anymore
UltraVNC/Chicken of the VNC
Transmission/uTorrent - bittorrent applications
Butler - application launcher for mac
BetterTouchTool - fixes the touchpad multi-finger gestures that OSX Lion fucked up.
SoundSource - easily switch between and control different audio devices from the menu bar
MassRenameIt - mass rename files
SubRip - use OCR to rip subtitles from a DVD into an srt file
ByteController - control iTunes with hotkeys or from the menu bar
Adium - mac port of Pidgin for IM
Sequential - comics reader, although rarely use it anymore since I read stuff on my iPad
ComicBookLover - iPad comics reader, free, good performance, decent features, uses app of same name on Mac to organize and add metadata to comic files (apparently an open format, so eventually other readers should be able to use it), the mac app isnt free, but the only reason you need to buy it is to get rid of a watermark when reading comics in it and a splash screen (but I use Sequential to read comics on my mac...so no reason, really)
AutoIt - Windows version of Applescript basically
CutePDF - create PDF files via virtual printer (why is this not built into windows already by this point?)
Joy2Key - use a gamepad to perform mouse/keyboard functions (used mainly for my media center, if for some reason im down a remote)
Unlocker - frees files from processes so you can delete them or whatever
VLC - even though it kinda sucks, I still use it a lot.