Although they have pretty much been superseded by CD-Rs and mp3s, cassette-only releases were once the only lasting documents of obscure scenes and artists who might otherwise have slipped under the radar entirely. At the time, of course, their circulation beyond a small group of fans was fairly limited (hence their enduring obscurity)... Now, however, some of those fans have dug up the old tapes and made them available to an almost limitless audience, via the web.
One of them is Krucoff, who has posted Lest We Forget, a compilation of 80's Berkeley punk bands, on his website (via Boing Boing). Another is Dutch net label WM Recordings, who have released a selection of mp3s from Bob Chaos, an 80's cassette-only label devoted to strange music from Muncie, Indiana.