Weight Weenies confirms that USE makes two of the lightest available in the general market -- the Alien Titanium and the Alien Carbon. I've got the Ti model; it's extraordinarily light at just under 150g and quite sexy. The site lists the carbon model at 104g, which I believe is a typo, but I could be wrong.
I thought the carbon model weighed 3 or 4 grams less than the Ti. Anyways, nice seatposts, readily available.
Thing is, there are probably boutique and custom component makers that can whip up parts of lower weight each day. AX-Lightness, the German company referenced in the Calfee link above, is primarily a custom parts maker.
You can probably line AX-Lightness up with a half dozen similar companies and say: "Ok... I weigh 130 lbs and am 5' 2". My butt is yea wide. I ride on pine needles and sometimes over dead snails. Give me the lightest seatpost that will hold me" ...and you'd get some interesting 36 gram seatposts.
Of course, that's a dangerous game, and also not one that satisfyingly answers the query, who makes the lightest seatpost? More interesting is, who makes the lightest production seatpost, and sells it on the general market without absurd rider weight limits?
That's the challenge... come up with products that set the bar for a reasonable range of rider weights. In terms of general useability, I'd point out that (at least in 2003) the Giant Mountain Bike Team used USE Alien Carbon posts on their rides...
Well, the guy asked for the lightest post, neither the strongest nor the most durable. I'm guessing he's aware of the trade-offs. I have heard great things about the USE Alien posts, but some complaints exist about the actual clamping unit breaking off, supposedly the post is real strong and so is the clamp, but its attachment of the two could use some work. Something they should look into fixing in the upcoming year/years. Still, I do agree that the USE Alien may be the best bet.
Yeah, fair enough. Just playing devil's advocate. Anyways, though, my point wasn't that those AX-Lightness posts were flimsy... they might be rock solid. It's just that the lightest custom post for, say, my little sister might not be the lightest custom post you could build for me... which takes all the fun out of the "what's the lightest post?" game.
The AX-Lightness stuff rocks from what I hear, though, and you're right -- their saddles are supposedly amazing. The commemerative Giant TCR 100 Team ONCE limited edition bike was spec'd with one; man, was that a neat little piece of carbon to cap off a stunning ride.