I listen to commentary tracks. I tend to like those that are anecdotal in nature- casting decisions, funny stories, etc. My favorites are:
Chasing Amy- too funny (the fan boy cut scene was great, too.)
Ghostbusters- more funny stuff- the MST3K motiff was neat also.
Star Trek TMP- it was interesting to hear about the Blaster Beam (the instrument used during the V-ger sequences) and all of the updates for the director's edition.