This is some of the software I use on an almost daily basis for my audio and video needs.

Winamp

The granddaddy of all audio players. Winamp plays almost every file type and has a wide array of plugins to extent its functionality. The most important plugin, for me anyway, is Titlebar which displays the title and time code of the currently playing track in the title bar of the window that has the focus. Even my work playlist has almost 10,000 songs so this is a must.

With the Shiny Classic Skin

Media Player Classic

Tiny size and plays anything (based mostly on the codecs you have installed - see below). Alt+Enter brings the window to full screen and playback is never choppy.

DVD Shrink (+ tutorial)

Extremely simple for backing up DVD's and also combining different video files from different DVD's.

DVD Decrypter (+ tutorial)

The Gold Standard in DVD ripping (and burning ISO DVD files).

K-Lite Codec Pack

I installed the "Full" version: K-Lite Codec Pack 2.72 Full (18.2 MB). K-Lite has any codec you'll need, audio and video, for playing/converting files found on the net. Be sure and install DirectVobSub also b/c it's needed for subtitles to show up in Media Player Classic.

AVIcodec

This is a wonderful utility that enables you to right-click any file (or folder) and display information related to those multimedia files including if you have the necessary codec required and if not, a download link to that codec. It also has a handy copy to clipboard feature that's copies all of the information for easy pasting into a forum post or email. It includes all of the essential information you need about the file, audio or video.

ID3-TagIT - THE ID3-Tag-Editor

This is important if you have a lot of MP3 files. It does ID3 Tag Versions 1.0+ and has some handy features like generating file names based on the tags and vice versa.

dBpowerAMP Music Converter

This is great for converting audio files to all of the important formats via a handy right-click menu. There are extensive codecs and extensions like the Audio CD Input which rips audio CD's.