You may know these already, but if not they are wicked useful. And free.
Switch audio file converter. For Windows or Mac. Convert audio files from many different file formats to mp3, wav or wma and more.
Audio Hijack. Record any audio. Freeware (for Mac) records for up to 10 minutes and is great for snagging movie dialogue to add to mixes etc. The old logo was much cooler...
VLC media player. Multimedia player and multimedia framework capable of reading most audio and video formats as well as DVDs, Audio CDs VCDs, and various streaming protocols. Put a copy on a USB stick and carry this free dvd player (also takes dvd screenshots).