ARCHIVES.ORG

Internet Archive has a mindblowing collection of recorded media, including films, TV commercials, and various audio files. Check out the fascinating Wayback Machine, which archives old websites (like this Gateway 2000 site from Christmas 1996).

You can also find some excellent stuff in the Live Music Archive. Most of the concerts up here are audience recordings of highly variable quality, although a few are from the soundboard (you can sample them before DLing them. I just got myself a copy of a Spoon show that I was at, back in 2003--pretty damn cool. Here are a few highlights from the many bands available:

SPOON (I was at this 4/9/2003 Irving Plaza show; I remember Joe Davis pointing out the Kinks cover to me.)
ELLIOT SMITH (This full band show from 9/28/2000 is excellent. On the other hand, this drunken 1/31/2003 show, recorded 10 months before his death, is pretty chilling and shows how far he'd fallen.)
DEATH CAB FOR CUTIE (Eddie Vedder joins them during a hometown concert in Seattle on 11/9/2004.)
THE FRAMES (This short but sweet 2/6/2006 soundboard show from the Dublin band sounds great.)
TENACIOUS D (You know what time it is? Tenacious D time, ya muthafucka, go! Fuck yeah! Check out this show from 9/28/2001 with Phish's Page McConnell joining them on keyboards.)