There's an introduction to HPGL at http://www.tobiastoft.com/posts/an-intro-to-pen-plotters. It's really easy, and you can get started a lot more cheaply than with AxiDraw.
Don't get me wrong, EMSL makes really awesome stuff. You just don't need to go for that kind of premium cutting-edge engineering in order to play with a pen plotter.
Will they plug into my mac via USB? And run off of a open source, easily available piece of software?
And if not, how much do you think those features are worth?
There's a whole community of us on twitter sharing our weird creations. Join us at #plottertwitter if you're curious: https://twitter.com/hashtag/plottertwitter?src=hash
You can turn 3D scenes into 2D vectors.
I made an egg plotter back in the early 2000s based on The Art of Motion Control site , and then I went off to build CNC tables and do wood carvings in 3-axis, and then 4, and 5-axis machining. I am really interested in the community around making ceramic 3D printers where they mix their own clay. All creative and really cool stuff!