We have an ongoing list of, but a lot of folks have requested a roundup of top battery cases for extending your iPhone's battery life, so here it is.
A few notes: While you should be able to go a day without recharging your iPhone 4 or 4S, some people use their phones more than others--or use battery-intensive applications such as video--and need more juice than what's offered with the iPhone's internal battery. Most, but not all, of the cases in this roundup are designed to double the battery life of your iPhone.
Many of the cases have batteries that are truly integrated into the cases while some have removable batteries that allow you to swap in additional batteries. And a few cases have detachable batteries that clip onto the back of the case.
Most of the cases try not to add too much bulk to your phone (who wants that?), but it's impossible to get around the fact that adding a second battery to your phone is going to add weight to it.
For those who can't deal with the idea of a bulked-up phone, another option is to buy an external battery charger (alas, we don't have a roundup of those yet). Some of those plug into the bottom of your iPhone, and others require you to connect a cable, which is a less elegant charging solution but one that a lot of people prefer.
Click on any image to see the picks (they're listed alphabetically).
Final note: We checked with manufacturers, and the cases in this roundup are all supposed to work with both the iPhone 4 and 4S. If you have suggestions for other top battery cases we may have missed, feel free to include them in the comments section below and we'll consider adding them as part of our next update. Also, if you have a favorite battery case that you're using now, get your 2 cents in.