Today’s smartphones are energy-sucking devices that can rarely go for a day or two without charging. I fondly remember my Nokia 6210 that could go a business week without needing a charge, but enough of the reminiscing.
If you do have a thirsty smartphone (or you get one for Christmas) and you find that the battery lets you down, you might be interested in Mugen Power Batteries, suppliers of replacement batteries for smartphones and other devices. Typically, a Mugen battery will offer an extra 10%-25% over the OEM battery of the same size. Take the battery for the HTC Wildfire S - the OEM is 1250 mAh but the Mugen version is 1500 mAh, giving an extra 22% more juice.
If you really need much more power, Mugen also sells batteries so big that you need a new back cover to fit it in. For the Samsung Galaxy S II, Mugen offers a 3200 mAh battery which nearly doubles the energy of the stock battery (1650 mAh), but the battery size increases too and a new back cover is needed. If you value talk time over aesthetics, it’s the only way to go.
Even Apple products are catered for, though as the iPhones et al don’t have user-replaceable batteries, the additional power has to come from an external unit.
Mugen are fairly well-known if you follow any of the main smartphone forums, and there are other vendors out there, but they seem to have the widest range with a good reputation, so if you need more power for your smartphone, check them out.