In the quest to extend the flight time of the AR.Drone David Homan has fitted a Zippy 2200mAh battery to his AR.Drone. This will take your flight time to roughly 30 minutes according to David. In his blog post (automatically translated) David explains that you need to take the extra weight into consideration while landing. He advises to bring the drone further down manually before hitting the landing button. He has some pictures in his blog post that show how he has attached the battery (which has a different form factor and weight) to his AR.Drone.
One of the first things you get annoyed by once you fly your Parrot AR Drone is the short flight time. Before you know it the battery runs out and you have to hook it up again. You can order some spare batteries from Parrot, but if you feel a little bit more adventurous you can order a battery from another brand with a higher capacity. The standard battery that comes with the AR Drone is only 1000mAh. According to the specifications this allows for 12-15 minutes of flight. Charging this battery takes some 90 minutes.
The 1300mAh battery used in the following video adds approximately 4 minutes to your flight time, but your mileage may vary. The video has a walk-through of how you can modify a custom battery if the connectors do not match.
Amazon sells the Parkzone 11.1V 1300mAh Li-Po Battery with EC3 Connector that is featured in the video for $32.