Linux up until the past few years has been left out of the emulators market as far as availability luckily this has change and now there are ports of almost every major system emulator including some emulators that are written strictly for the linux operating system, the following site has a nearly complete list of all emulators that are available through the Ubuntu package manager and what repository they are in, The site is also slowly adding source files that can be compiled on any other linux distribution.

The list: http://retro-gaming-world.com/emulators-for-ubuntu/