If you value your Dell Streak's battery life more than its slim profile and you don't want to carry around an external battery pack, then Mugen Power has something you'd be interested in. Like it does for many other devices on the market, the company has made a massive extended battery for the Streak that significantly increases the gadget's thickness and requires its own battery door/cover (included).
Mugen's 4800mAh li-ion battery is more than three times the capacity of the stock 1530mAh battery, but don't be surprised if you don't get three times the runtime. Independent tests have shown that third-party batteries from Mugen, Seidio, and other companies aren't always what they seem, so take that into consideration before you pay $129.95 for this. There's also an 1800mAh battery ($39.95) that's the same size as the stock battery, which means you theoretically get more capacity without having to use a new battery cover or alter the Streak's appearance. I use a similar battery from Seidio for my HTC EVO 4G and have no complaints, but your mileage may vary.
Update: Video of both batteries below.
[Mugen Power] Thanks, anonymous!