Personally I have been using the Firestorm Viewer for about as long as it existed and it offers a nice set of additional features over the default viewer but I regularly hear that others use a different third party viewer. What do you use and why?
Firestorm all the way for my daily driver, though once Alchemy catches up to all the QOL features that FS has, I'd love to switch. 120FPS goodness, but still fairly basic and the trade-off isn't quite balanced for me yet.