I find that I need a memory refresher!  I do, indeed, know what constitutes music a cappella...music performed with only the human voice as instrument.  Some of the songs I posted here are NOT a cappella at all.  I can only plead that when thinking of these songs I remembered only the vocal parts and not the instrumentation that accompanied them.  Still good music, still amazing vocal arrangements and performances, but most definitely NOT a cappella.  oopsie...
And now, for your listening pleasure...
I love music a capella.  There is a TV show coming for another season called "The Sing-Off" that is all music made with just human voices, no instrumental accompaniment...I think I'm going to love it!  Here are some singers I admire and some songs they've sung a capella .

James Taylor, of course:

 

Is That the Way You Look? Click on the linkie thingie there to the left. I couldn't find a video of Taylor performing this song, but found this very silly video of a couple of kids 'dancing' to it. The song is not entirely a capella, but still great.

and another James Taylor: 

Noticing a theme here? What can I say? The man can sing!  

A Huey Lewis cover of a classic hit:

I've been playing that one in my car a lot lately...very, very loudly!

And one from Billy Joel:

Be aware that my dialup Internet connection prevents me from previewing any of these links, so I hope that I'm giving you the right ones! :D  Enjoy!