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?
For people using MacOS, Ubuntu, Debian, Fedora, OpenSUSE Tumbleweed, or FreeBSD, Megapahit viewer is a nice choice for anyone wanting a mostly-vanilla Second Life experience with a few modest tweaks and added features.