I'm getting started in the ultralight backpacking business. In fact I've become strangely obsessed with it and I've put in several days worth of internet research and mental obsession. My target trip conditions are serveral days hiking (not climbing) during summer, in high country. Since I've gone through several mental iterations I thought I might as well share my conclusions.
My gear list shows the choices I made and the associated weights. I weighed 'em using a kitchen scale with resolution of 1/8oz.
Key philosophical mindset points:
As I see it, a shelter protects (to some degree) you from four things, in descending order of importance/effectiveness: precipitation, bugs, wind/cold, and an exposed feeling (especially for city slickers).
Initially, I splurged on REI's lightest solo tent, the Mountain Hardware Stiletto 1, rather than the cheaper, marginally heavier REI Quarter Dome T1, before I realized it's still kind of heavy, compared to other options.
The real ultralighters favor tarps or tarp shelters (i.e., no floor) at as little at 5oz, with head/bivy netting for bugs, or even just DEET. You can save a pound or more this way but may have a less sheltered feeling.
For about the same weight you can get a single-wall shelter (aka "tarp tent") instead. The three that looked best to me are Gossamer Gear's "The One" (which is indefinitely not in stock), Tarptent's "Contrail", and Six Moons' "Lunar Solo Enhanced". Of the two you can actually buy, they're pretty much a tie, but I chose the Contrail for price, aesthetics and apparent ease of use.
The Tarptent is a little less flexible than a normal tent or a tarp/net combo - it's all or nothing. But the protection is more complete vs. cold and bugs.
Down is lightest and smallest, but you can't get it wet. I chose down and I'll just keep it dry.
Here's one area where, as a cold, creaky-boned side sleeper, I was willing to compromise a couple ounces for comfort. If a closed cell foam matress will work for you, Gossamer Gear's or Thermarest's will do the trick in around 8 oz (with an R-value around 2.2 - 2.5). But for 10.2 oz Mont-Bell's UL 90 pad should have an R value closer to 3.5 and be more comfortable. It's short so the legs go on top of the (mostly) empty pack. And it takes much less pack space.
Closed cell foam pads can be trimmed down, quite a bit though.
It may not even be neccessary, but Gossamer Gear's Polycro or a sheet of Tyvek is only 1.5 oz. It's the thing to do.
For a shorter trip I'm going to try a Vargo Triad (1 oz) with denatured alcohol. The weight of the fuel dominates, so for a longer trip perhaps the Snow Peak LiteMax is a better, less ecological idea. The weight of the Trail Designs Caldera Cone should be easily made up for in fuel weight. The one I ordered won't arrive in time so I made my own out of aluminum flashing. UPDATE: It did show up and is lighter-gauge aluminum that mine. Still, it weighs almost as much, and more with the bag it comes with protecting it.
I'm using REI's 1.3L nonstick Ti pot w/ lid. UL folks seem to be happy with 0.9L pots, so maybe that's a poor idea. I bought a Ti bowl too, but I've realized that's a vast luxury. Instead I'll eat out of the pot or one of those sturdy Ziplock bowls.
In future I think I'll try the Snow Peak Titanium Trek 700 Mug and experiment with differnt wind screens.
And you can go stoveless, but to me, that's just no fun.
A 3G iPhone weighs 4.7oz and serves as a phone, camera and GPS to some degree (kept turned off most of the time). For a really long trip, or if you want to listen to music, the Solio solar charger is a lamentable 5.6oz.
For picutres better than 2MP, the 10MP Casio Exlim EX-Z80 is cheap and as light a digital camera I could find: 3.5oz. But are the panoramas on any digital going to be that satisfying? A film camera is probably a better quality-for-weight option; I don't know which would be best. I think I'll just bring a disposable to supplement the iPhone.
Boiling water to purify it isn't efficient due to weight of the fuel. The Steripen Adventurer (3.6oz) looks like a good choice if it weren't for the tablets option: Katydin Micropur (or equivalent) tablets are vanishingly light, work great, and taste okay.
Platypus bags are much lighter than Nalgene bottles. Two 1L bags will do. (Among hard water bottle, the white HDPE ones are lighter than the fancy-looking ones.)
If it would fit, the Gossamer Gear Murmur is incredibly light. Maybe next time. A larger Gossamer Gear pack would work, or the GoLite Jam2, which is what I got.
Part of the point of ultralight hiking is that you can wear light trail running shoes rather than heavy boots.
Camp shoes are thus less coveted, and good thing they are: shoes are heavy, even Crocs. Just loosen your laces - instant camp shoes.
Some go commando; not me. With two pair of synthetic you can wear one while the other dries. The extra pair is a bit of a luxury - you could work around a one-drawers plan, especially since there's always your long johns. Maybe I'll get there someday.
I bought a cheap Walgreens brush (your dentist recommends soft bristles) because it looked lightest among those at the store. Then I tore off the rubber grippy strips and drilled out the handle and sawed it shorter. I think I did all this as a joke, but I'm not sure. My crappy scale calls it 1/4oz.
Possible other items I considered and rejected: reading material (why not enjoy the outdoors, rain or shine?), a footbag (solo anyway & too tired for such antics, or should be hiking), monocular (might be worth having - I didn't research 'em), GPS (good survival thing also needs research), and whiskey (but might as well live naturally).
Ultralight Joe had some good discussion, links, weights, etc.
Adventure Alan is ridiculously light.
More good info at backpacking.net and Backpacking Light.