Several individuals have asked about which pack to buy for given purposes.
Lets make a quick canvas of Summit viewers picks for "best" backpacks. I
broke backpacks in to three categories. 1. Weekend to seven days 2.
Extended to Expedition 3. Technical (Rock Climbing, Ice Climbing, Extreme
Skiing) . Consider these factors: Features, Quality, Comfort, Cost, Versatility, Wear Longevity, Company Tech Support. Feel free to add a comment of why you made your picks.
Hopefully there will be some backpacks that rise to the top in each category.

Forgot to add two categories Male & Female to the above post.

I'm a huge fan of my 38L Osprey Mutant.

I love the helmet carrier and the spots for my ice axe/ice tools, and the removable lid is fantastic too.

The hip belt is a little small, but it has ice screw holsters and you can actually turn the hip belt into a "minimalist" hip belt which allows for more hip/leg movement.

I got it at Wilderness Exchange in Denver for $95 and it has Osprey's guarantee.

There are also larger versions.

Edit: I would not use this for treks where larger than 38L is needed.

1. Cilo Gear 45L Worksack
2. I currently use a Gregory Whitney that is a little to big for me. I'd prefer something lighter with fewer pockets and straps but the selection of 100 liter packs is pretty small.
3. It really depends. I use a Metolius haul bag for rock climbing and a Black Diamond Zippo 27 for ski/snowboard day trips. No ice climbing for me, yet.

Skiing Specific Pack
Just got the new Mammut Spindrift 26 and have used it several days.
Perfect size for day trips or sidecountry.
-Love the HUGE hip belt pockets.
-Very nice avy gear storage.
-Light without feeling "delicate".
-Like everything from Mammut, it's well thought out; features like helmet storage, hydration, and adjustability are done to perfection.

1. Deuter Guide 45
2. Gregory Massif or Deuter Contact 65+
3. Deuter Glide 35 or Osprey Mutant 28

I love my Osprey 80 pack for extended trips.