I was led to this site [NSFW] from an effective bit of spam. I was particularly impressed by the animations here. I love the names of the various positions. I think $190 is a bit steep for such a simple thingy though. I would love to have been a fly on the wall when the inventor hit upon this concept.

UPDATE (12/06): if you got here doing a Google search for the Gravitizer, the site I linked to originally doesn't work anymore. Here's another company (link removed -- see below) offering what appears to be the exact same thing (with a very similar list of crazy sex positions accompanied by the weird little bouncy animations [NSFW]). There are others out there too, but these guys offered me a free chair if I updated this post with a link to their site. If and when I receive the chair, I'll have a contest at Candleblog of some kind and give it away (unused, I promise!).

They never sent me a chair so I removed the link. Don't buy their product anyway, they're a bunch of liars.