A lot of good suggestions have been added, and I need to underscore that, like backpacks, no one tripod is the best for all uses. I own an ultra-light expensive Gitzo setup with a Really Right Stuff ballhead and plate set (tall, rock solid), a cheap Slik aluminum jobbie (shorter, more wobbly, packs smaller), and a Gorillapod SLR Zoom (teensy, lightest, most fiddly, worst in winds). Each really has its own uses, strengths, and weaknesses. I find the Gorillapod the least useful above treeline, the Slik best for good-weather short treks, and the Gitzo when the weather/terrain may be challenging and photography is one of the main goals of the trip. Heck, I even have the tiny original Gorillapod when I just want to take my Lumix TS3 (I sea kayak as well as backpack), which is a decent combo if all I want are hands-free snapshots, as opposed to a full Canon 5D setup with extra lenses.
YMMV and horses for courses, but wanted to reflect that a lot of the suggestions in this thread are equally valid, it just depends on the kind of shooting you do, where, and in what conditions.