My best friend has been my Ncharge battery from Valence.

It's an 8.5"x11"x.5" unit that plugs into my laptop's power port and delivers "plugged in" power for about 10 hours of DVDs, etc. It's also wonderful for meetings, etc., where you wish to minimize cord clutter -- and not worry about your laptop going into "power conservation" mode while you are chatting with clients.

Steve