They are The Feynman Lectures on Physics.
It is not all that uncommon to hear students quote from the Feynman Lectures in biblical fashon, quoting book and verse:
"Oh, you want to know how I did that least action problem? Check out Feynman, Book 2, 19-5."
Students use the red books to study for qualifiers, or for their GRE's (and perhaps that is a mistake).
Other physicists make a once a decade pilgrimage to spend a significant portion of their free time reviewing the red books cover to cover.
Why this devotion? Because the Feynman Lectures on Physics is perhaps one of the greatest collection of books on general physics that exists!
The classes the lectures were based on were by all accounts a nightmare for most of the regularly admitted students. The courses were an experiment, and a work in progress. However, the classroom where the lectures were given was always full, as in standing room only. Most of those in the class were grad students or other professors auditing the lectures. You see, the lectures are best appreciated after one has already made it through the gauntlet of an undergraduate physics curricula. The books are not good for learning physics for the first time, but are wonderful to come back to and font of wonderful insight and nuggets of wisdom.
Ahh the internet! What a wonderful resource! The generation before widespread use of the web were lucky indeed if they ever had a chance to view a Feynman lecture. It was a special treat when someone would bring a reel to reel or a VHS of a Feynman lecture. I heard tales from my teachers of the beer parties where someone would produce a beat up and well played VHS or Beta of a Feynman Lecture.
Now days, it is rather easy to "bittorrent" a copy of almost all of Feynman's works, be it written or in Audio or Video.
The ease of getting these gems on the net in no way detracts from their power. Feynman had a powerful way of presenting ideas, and in presenting physics. Perhaps only Carl Sagan rivaled Feynman in clarity and insight. But Feynman was first, and as much as I like Sagan, in my opinion was the better at "explaining things."
Bill Gates recently announced his Project Tuva. He rhapsodizes on his goal of bringing science and physics to the masses by making Feynman's Messenger Lectures freely available to the masses.
These lectures have long been available through sharing sites, or torrents, but it is nice to be able to view a legit copy, with captions!
Some Google Fu will tell you why the website is named Project Tuva. For a more interesting explanation, read Surely you're joking Mr. Feynman.