Update: Found something even better and it's also on Pandora's blog: A page that lists examples (from pop/rock music) of some of the different kinds of harmony.
I just discovered that Pandora has a blog and it even carries the occasional music lesson! I found this excellent video primer on harmony singing on the blog.
What makes the video really cool is that it takes one simple melody line and applies different harmony techniques to it: tutti, "drone", unison, counterpoint etc. You need absolutely NO knowledge of musical theory to enjoy the video.
If you prefer a link to the page, go here.
I also found a very interesting page on harmony singing for Bluegrass music. (For the uninitiated, this page definitely needs some rudimentary knowledge of musical theory.)
I often wonder why harmony (especially counterpoint) was ignored by classical musicians during the early days of music development in the Indian subcontinent? Though one of the delights of Indian classical music is hearing the tanpuras's shimmering Sa and Pa (the classic interval of fifths; C and G notes) forming patterns with the notes of the raag/composition.