Those of us with smartphones can admit that while they're super useful, they're not very battery efficient. So how can we stay powered when there isn't always an outlet nearby? Three words: Energi to Go. This is Energizer's line of mobile power packs, made by XPAL. I know I usually talk about Android with you all, but the Energi to Go line is available for many other phone makes and models. They are essentially battery packs that you can put in your pocket, laptop bag, or glove compartment (although they make some larger ones, too).
My favorite of the series, so far, is the XP4000. This is a 4000 mAh battery with one USB port and a one-press battery meter located on the top. The device comes with a USB cord that has an adapter on one end, and can connect to many different phones. For instance, the most popular right now would be micro USB, which is an adapter that is definitely included. However, you can also plug the USB charger that came with your phone into the battery pack and it will work just the same. Take notice that this particular pack is actually made with digital cameras in mind, but since I sometimes forget to charge my phone, it's nice having roughly 2.5 charges worth of portable power (I'm still using a Nexus One). A close second favorite for me is the 4001, which is similar to the 4000, but with two USB ports so you and a friend, family member, or spouse can share one Energi to Go power pack. There's no battery meter on this model, just a check-battery light, but it does just fine.
For those of you who might want something a little smaller, you could go with the XP2000. This can be purchased in a kit that includes an AC adapter, car adapter, and the same standard USB cord and interchangeable tips. If you still need something smaller... there's the AP650 Energizer Energi to Go Energi stick (say that five times fast), which can provide up to 90 minutes of talk time. This is currently only available in micro USB format. Also, they are rechargeable by way of your phone's micro USB charging cable. The best feature though: they fit right on the phone so you can talk on it while it charges. If you keep one of these in your pocket, try to remember not to sit on it—I assure you it isn't very comfy.
Energizer and XPAL haven't forgotten about iPhone 4 users, either. They make the AP1201 which is a silicone case with a charger built into it. It has different charging modes so you can charge the case and phone at the same time, or charge the phone first and then the case. Additionally, there is a built in bumper to help with the initial antennae interference issues. If you're on 3G, you can get up to an extra 5 hours of talk time with this case/battery combo. You can also get an extra 27 hours of music... that should hold you over til you can get to an outlet, right?
Overall, I find these battery packs by Energizer and XPAL to be the most useful accessory I have for my phone. The portable power packs can be used for charging any device that can use USB, making them great for traveling, but also just for having in my car when I forget to charge my phone.
If you'd like to see their whole collection of portable power products, you can check them out here.