While looking for an online drum machine that would make it simpler to give examples of rhythms on this blog, I stumbled across ZeFrank's sequencer (old link, via O'Reilly).  This is a lot of fun to play around with, partly because of the unexpected sounds, but also because you can slide the timing of each sound around within the bars.

This is a great way to test the ideas of microtiming - how you can shift a beat in ways too small to count in traditional notation, but still feel the difference.  For a good audio example of how that works, check out the video of Burial's "Untrue".....

(Thanks to Greg Swindle for introducing me to the Burial track.  His downtempo remixes are what I listen to on the train in order to meditate on the way to work....)