MAME

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MAME is a multi-purpose emulation framework. The acronym originally stood for Multiple Arcade Machine Emulator.
MAME’s purpose is to preserve decades of software history. As electronic technology continues to rush forward, MAME prevents this important "vintage" software from being lost and forgotten. This is achieved by documenting the hardware and how it functions. The source code to MAME serves as this documentation. MAME at some point absorbed the sister-project MESS (Multi Emulator Super System), so MAME documents a wide variety of (mostly vintage) computers, video game consoles and calculators, in addition to the arcade video games that were its initial focus.

I must say the first time that I ran the Snow Bros arcade game on MAME and hit that '5' button on the keyboard to hear the clinging noise of coin inserted to the game... it kinda felt like I was stealing! Boy was it fun playing that game on my computer. It was unbelievable. Then I saw how many games were already supported and couldn't believe that either. To this day I find new (well not new.. old) games that I (may o may not) had played before in the arcade to get my nostalgia dose out of. 

Fun fact is that there's actually a Java port of MAME called arcadeflex which is actually still under development and has emunewz.net as it's home.

MAME Official site