RetroArch is a collection of emulators (or "cores") for the PS3. It can be obtained by their official site RetroArch.com. A list of emulators on the PS3 that aren't in RetroArch can be found here. Newer versions support NetPlay, which allows for multiplayer play over the Internet.
RetroArch can be used in conjunction with RetroXMB PKGs for displaying a direct launchable ROM on the XMB.