Sherpa Sport Messenger Pet Tote. This is low profile (I like it best in black) and very functional. There's an outer zipped pocket and a bigger pocket with organizer pockets for cell phone, ipod, etc. There's also another pocket inside the tote. These extra pockets are key--I've seen many pet totes that have no pockets or one inadequate pocket forcing you to have to carry another bag. With this, you don't need an extra bag.
There are two vented pockets on either side, and an optional one in front. The bag comes with a removable fleecey insert for easy cleaning and comfort for your pet.
It's airline approved and would be great under your seat, although they do come in larger sizes and I'm not sure if all sizes are approved, so be sure to measure. This bag works great for my 5lb Chihuahua. She loves to come with us everywhere we go and people don't realize she's there unless we tell them. If you need something for the holidays and beyond, I highly recommend this bag and think it's a great value. Find it HERE.