If you're looking for a size small (if you're up to 5'6" or so) lightweight Niner this is the deal of the century. Seriously. It weighs in at just 18 lb 5 oz. Granted it's set up as a single speed right now (with a Paul Melvin to pick up the chain slack) but even if you add gears this will be a light weight bike.
It has a Scandium frame, a carbon Niner fork, Chris King hubs and headset, and a Thomson post and stem. But best of all it's being sold for just $1300, and it's in mint condition.
If the Air Nine isn't what you need then check out our other used bikes listed below:
Trek 9.9, $1500, XL
Surly Karate Monkey, $800, 20"
Surly LHT, $1000
Redline d660, $2000, XL
Trek 2300, $690, 58cm
Niner Air Nine, $2850, XL
Giant XTC Two x One, $599, 19"
Trek Carbon EX, $2800, L