(This is probably more information about external batteries than you really wanted, but here goes anyway)
I currently have three external / portable batteries that I use with my gadgets.
I started off 4 years ago with an APC 10WH battery I bought for around 40 dollars at Staples. It gave me about 2-3 recharges for my 60gb iPod video 5th gen. It only came with a USB to mini-USB cable in order to charge the battery itself from a PC.
The APC will charge my myTouch 4g at least one full charge, then maybe another half charge, and would add power to a Droid X. After almost 4 years, it still works, and I like the styling of it, for what that's worth. Just bring your own cables for your device...
I bought a 3 foot long, right-angle micro-USB to USB cable for around ten bucks from Amazon so I could coil the cord and charge the phone in a jacket pocket. This also worked well for me, not having the microUSB cable sticking out of the side of the phone. HIGHLY recommended!
Last summer I bought an Energizer XPAL 2000 MAH battery 'kit'. It came with a spiral-coiled cord and 'tips' to use for a variety of phones. (mini-USB, micro-USB, Samsung, Nokia, Apple, etc) I bought this for 35 dollars on Amazon after we had a three day power outage at work and everyone asked to use my APC charger to top off their phones.
The XPAL 2000 will charge a myTouch 4g or iPhone 4 about one full charge from 20% and then a little boost afterwards. It wouldn't charge my buddies Droid X for some reason. The battery is a little smaller than the APC unit, and shaped like a smaller myTouch, in that it's curved a bit on the corners. It's lightweight, and should work to double your battery capacity. It also came with a nice cloth pouch to hold the battery, cable and tips, and it can be charged from any powered USB port.
Then I went nuts and spent 55 dollars on a larger Energizer XPAL 4000 kit. This battery also came with a pouch, a spiral-coil cord, even more tips, and a clip thing that might be used to charge certain types of camcorder batteries.
I was able to charge a myTouch 4g, a Droid X, and top off a BB 9650. The thing has alot of juice, but it's pretty big to carry in a pocket, being about the size of a Droid X or an Evo 4G. One major disappointment is that you need to charge it from an AC adapter instead of from a USB port. If you go on vacation, that is one more AC adapter you will have to carry with you.
Then I went totally insane and bought a spare HTC battery for the phone for under 30 bucks. What can I say, I like overkill.
Overall, I'd say to get the 2000 or the APC if it's just the phone and maybe an iPod you care about, and get the 4000 if you are a generous soul who likes to share your spare electrons. And although the XPAL units came with cords, I almost always use the right-angle USB cable I got separately.
APC Universal Battery, 10WH (Amazon.com)
Startech right-angle MicroUSB to normal USB cable (Amazon)
Energizer XPAL 2000 smaller battery 'kit' (Amazon)
Energizer XP4000 larger battery 'kit' (Amazon)
HTC & Seidio internal batteries for your myTouch 4g (Android Central)