A hack I did for musichackday Boston 2010:

synchronize a slideshow to a soundtrack, using EchoNest API to find beats.

Example video:
My excellent day in Provincetown

The number of slides is the same as the number of beats, so it shows how well the EchoNest beat finder did on this particular audio file.  Near the end is pretty good.

To use:

First: repeatedly run slideshow, a perl program.

Then: Launch QuickTime7; File>Read Image Sequence @ 24/sec; add soundtrack

Music hackday link: slideshow sync

Sample terminal session:

The script is run with the same args each time: a folder of pictures, a audio file, and an optional folder to put the movie frames. 

The 1st run uploads the audio; the 2nd run downloads the beat data and creates the frames.

 slideshow.pl <pixdir> <audiofile> <framedir = ~/Movies/_frames/>

Arranges slideshow timed to soundtrack, using EchoNet API to find
beats. An image sequence is prepared to be read in as movie. 2
support files are created in the working directory.

for musichackday Boston 2010, by N. Resnikoff (nedwaves.com)

>bin/slideshow.pl mhd/images/ mhd/soundtrack.mp3

Uploading file..
..md5 hash written to /Users/ned/mhd/soundtrack.md5

>bin/slideshow.pl mhd/images/ mhd/soundtrack.mp3 
Analysis not ready at ./slideshow.pl line 144.

>bin/slideshow.pl mhd/images/ mhd/soundtrack.mp3 
Found 56 image files.
L(7); L(13); L(17,18); L(24,25);
Finished; read frames in at FRAMERATE=24/sec