I use for audio: iTunes & Audion.
Video: VLC & at times QuickTime Pro.
Audio Streaming: Both VLC & iTunes.
Video " " : Depends on format. It might include wmp, vlc, rp, qt, etc.

Again, I may use a range of players for different purposes, including some not mentioned here. eg: mplayer, wire tap, audio hijack, etc.
blah blah & converters or go-betweens.

For windows: dBpowerAMP, WinAmp, Quintessential all have their merits!!! QCD being open source which is a major plus & should be supported. LW is open source remember, as is vlc, mplayer, audacity, etc.