Alternative Launchers and Frontends
skeletonKey
skeletonKey is the successor to retroInvader. In addition to a simple netplay interface, nearly every aspect of retroarch can be set using this GUI.
retroInvader
retroInvader is designed to allow users to quickly install & configure the latest version of RetroArch, cores, and shaders. In addition to the GUI-driven configuration & installation, retroInvader supports drag and drop for quickly launching ROMs, hosting netplay sessions as well as installing BIOS files. retroInvader is for Windows only.
RAEM
RAEM (formerly RA-Player) is another launcher for RetroArch, for those that want to try something that looks more like a native Windows program. It hasn't been updated in a while, though.
Phoenix
Phoenix is an alternative frontend for libretro that is independent of RetroArch, not to be confused with the discontinued RetroArch-Phoenix launcher. It is intended to mimic the style of OpenEmu, except using libretro and being available on Windows and Linux. It is still in early development and requires that you build it yourself through Qt, though the main developer says an alpha release with prebuilt binaries is on the way.
Minir
Minir is another alternative libretro frontend that is independent of RetroArch and inspired by bsnes-Qt. It is stated to be a complement to RetroArch, focusing on things that RetroArch does not focus on, such as a system native GUI, cheat search, debugging, etc. This frontend is still in early development and no binary builds are provided yet, but you can build it yourself from the source code on Github.