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.

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p>[Update:  I'm fixing up this post that rambled off in too many directions - a victim of late night posting.  This post deals with the "Swing Pairs" - similar rhythms that shift between 12/8 and 4/4.  See here for the exercise and chart of the comparison of rumba clavé and 6/8 clavé....]