Currently, I use a Slappa Velocity Spyder laptop backpack as my "under the seat in front of you" bag. It is a well sized bag (21"x13"xX") and it fits under well enough that I rarely get questioned about it and even then if I nudge it with my foot they let it go. It's not the prettiest thing in the world but you can cram tons of stuff in it.

http://www.slappa.com/velocity-spyder-laptop-backpack

However, it is starting to show its age on the inside (the outside would survive thermonuclear war without a scratch) so I am looking for a replacement. It has served me well in travel and work and if I can't find something better I will buy another but it pays to see what is out there.

Generally, it contains:

- A 15" laptop
- Charger/cable for my laptop, cell phone and camera
- A D-Link three outlet adapter
- A book
- A small consumer grade digital camera
- Cell phone
- Keys (having a dedicated key chain holder would be great!)
- A sweatshirt
- Small food, Trader Joe's cereal bars or similar
- 1 liter water bottle
- Pens, assorted change, other small items

I know there are lots of bags that could easily swallow up those items and that the space for under seat luggage is not consistent. However, I am curious what you folks use.