One of the drawbacks to buying an extended battery for your HTC EVO 4G is that there are hardly any cases for them. These batteries require a replacement back cover to accommodate their extra thickness and not all companies use the same one (e.g., Seidio, Mugen Power, and Sprint's battery doors are all different).
Third-party case manufacturers obviously don't design special cases for each one, so users are often forced to choose between having an EVO with more runtime or having an EVO with a case. There are cases for Seidio's 3500mAh Innocell battery (the Innocase Extended and Innocase Extended Rugged), sure, but that's only because Seidio makes them. And not everyone has/wants an Innocell, especially when there are other 3500mAh batteries out there that are much less expensive.
Such batteries may be from companies most people have never heard of, but they're really cheap, they could be of decent quality, and a lot of them use the same battery door (the one you see here, here, and here). And as it turns out, there are actually red and black silicone skins available for these extended battery/cover combos too.Amazon (black) | Amazon (red)] Thanks, Carl!