What is RetroArch?

RetroArch is a modular multi-system emulator system that is designed to be fast, lightweight, and portable. It has features few other emulators frontends have, such as real-time rewinding and game-aware shading.
What platforms does it support?

RetroArch sets the standard on cross-platform portability right now.

Linux (x86 and x86_64)
Windows (32bit/64bit)
Mac OSX (PPC/Intel)
Playstation 3 (PS3 SDK/PSL1GHT (WIP))
Xbox 1 (XDK)
Xbox 360 (XeXDK/Libxenon)
Wii
Raspberry Pi
OpenPandora

Why is it special?

Right now RetroArch is unique in that it runs the same emulator cores on multiple systems (such as Xbox 360, PS3, Wii, PC, etc).

For each emulator ´core´, RetroArch makes use of a library API that we like to call ´libretro´.

Think of libretro as an interface for emulator and game ports. You can make a libretro port once and expect the same code to run on all the platforms that RetroArch supports. It´s designed with simplicity and ease of use in mind so that the porter can worry about the port at hand instead of having to wrestle with an obfuscatory API.

The purpose of the project is to help ease the work of the emulator/game porter by giving him an API that allows him to target multiple platforms at once without having to redo any code. He doesn´t have to worry about writing input/video/audio drivers - all of that is supplied to him by RetroArch. All he has to do is to have the emulator port hook into the libretro API and that´s it - we take care of the rest.

Supported game / emu cores
bNES

Libretro port is: A Nintendo Entertainment System / Nintendo Famicom emulator.

Libretro author: Themaister

Original author(s): byuu

Compatible with: RetroArch PC

RetroConsole Level: TBD

Github: https://github.com/libretro/bnes-libretro
bSNES

Libretro port is: A Super Nintendo/Super Famicom emulator.

Libretro author: Themaister

Original author(s): byuu

Compatible with: RetroArch PC

RetroConsole Level: 3 (Performance/Compatibility), 4 (Accuracy)

Github: https://github.com/libretro/bsnes-libretro
Final Burn Alpha

Libretro port is: A multi-arcade system emulator.

Libretro author: Twinaphex, Themaister

Original author(s): Various

Compatible with: RetroArch PS3, RetroArch 360, RetroArch PC

RetroConsole Level: TBD

Github: https://github.com/libretro/fba-libretro
FCEUmm

Libretro port is: A Nintendo Entertainment System / Nintendo Famicom emulator.

Libretro author: Twinaphex, Themaister

Original author(s): Various

Compatible with: RetroArch PS3, RetroArch 360, RetroArch PC

RetroConsole Level: 0

Github: https://github.com/libretro/fceu-next
Gambatte

Libretro port is: A Game Boy/Game Boy Color emulator.

Libretro author: Themaister, Twinaphex

Original author(s): Sinamas

Compatible with: RetroArch PS3, RetroArch 360, RetroArch PC

RetroConsole Level: 0

Github: https://github.com/libretro/gambatte-libretro
Genesis Plus GX

Libretro port is: A Sega Genesis/Mega Drive, Master System, Game Gear, SG-1000 emulator.

Libretro author: Twinaphex, Themaister

Original author(s): Charles McDonald, Ekeeke

Compatible with: RetroArch PS3, RetroArch 360, RetroArch PC

RetroConsole Level: 1

Github: https://github.com/libretro/genesis-next.git
iMAME4All

Libretro port is: A multi-arcade system emulator.

Libretro author: ToadKing

Original author(s): Nicola Salmoria, Various

Compatible with: RetroArch PC, RetroArch PS3, RetroArch Pi

RetroConsole Level: 0

Github: https://github.com/libretro/imame4all-libretro
Mednafen PSX

Libretro port is: A Sony PlayStation emulator.

Libretro author: Themaister

Original author(s): Ryphecha (Mednafen)

Compatible with: RetroArch PC

RetroConsole Level: 4

Github: https://github.com/libretro/mednafen-psx-libretro
Meteor

Libretro port is: A Game Boy Advance emulator.

Libretro author: Themaister

Original author(s): Philippe Daouadi.

Compatible with: RetroArch PC

RetroConsole Level: TBD

Github: https://github.com/libretro/meteor-libretro
NXEngine

Libretro port is: A Cave Story/Doukutsu Monogatari engine.

Libretro author: Twinaphex, Themaister

Original author(s): Pixel, rogueeve

Compatible with: RetroArch PC

RetroConsole Level: TBD

Github: https://github.com/libretro/nxengine-libretro
Prboom

Libretro port is: A Doom engine.

Libretro author: Twinaphex, Themaister

Original author(s): id Software, Various

Compatible with: RetroArch PS3, RetroArch 360, RetroArch PC

RetroConsole Level: 0

Github: https://github.com/libretro/libretro-prboom
SNES9x Next

Libretro port is: A Super Nintendo/Super Famicom emulator.

Libretro author: Twinaphex, Themaister

Original author(s): Various

Compatible with: RetroArch PS3, RetroArch 360, RetroArch PC

RetroConsole Level: 1

Github: https://github.com/libretro/snes9x-next
VBA Next

Libretro port is: A Game Boy Advance emulator.

Libretro author: Twinaphex, Themaister

Original author(s): Forgotten, VBA-M Team.

Compatible with: RetroArch PS3, RetroArch 360, RetroArch PC

RetroConsole Level: 2

Github: https://github.com/libretro/vba-next

The company that was doing the shared hosting went bankrupt, apparently, and they didn't provide any database backups or anything before they went down. ZachThibeau has some new hosting setup, and he and thomas_sch are attempting to load a Google(?) cache scrape of the old forum's posts into a new forum framework. The forums should be back soon, but they will have a new location, http://libretro.com/forums. I assume the old forum address will redirect to it, once it's up and running.