i heard this last month while writing about electronic music in Iceland. although it's not quite the strangest music i'Ve ever heard (there are numerous comparisons with Stockhausen), it's credited as the 1st ever electronic music peice composed in Iceland. When it first played in Iceland in 1960, it damn near caused a riot amongst the highly conservative chattering classes.
Here's some entries that may qualify as strange, in different ways:
Arthur Brown - Fire
Fractal music, Mandel based math; recursive patterns. Unknown composer. I like this piece, tonally, more than some of the other similarly generated compositions people have made.
Alarmingly insane, utterly wonderful granular audio/video installation performance at STRP called "Model V".
Current 93 (David Tibet)... Awesome neo folk. Great lyrics.