Websites on Acoustic Phonetics and General Acoustics
Praat - A speech analysis program (for just about any platform), created by Paul Boersma and David Weenink.
Monthly Mystery Spectrogram Website - Includes several good examples of spectrograms and a brief tutorial on how to read them. There haven't been any new mystery spectrograms posted in a while, but there is an archive of old ones, presented with detailed segment-by-segment commentary. Maintained by Robert Hagiwara.
Sound Waves - The first page in a series of course notes on acoustic phonetics, from a course taught by Kevin Russell at the University of Manitoba. You may recognize some of the diagrams from Rogers's textbook.