GameTap is hella cool
I found www.gametap.com tonight and signed up for a two week free trial. In a nutshell its this. You pay $9.95 a month to get access to 500+ games (and growing). You select the game you want to play and it is downloaded to your system and cached. You then play.
What's the cool part? They have all types of systems: DOS games, Sega Genesis, Atari 2600, Arcade game, Dreamcast, Intellivision, etc...
I have already played the original Zaxxon, Kings Quests 1, Zork 1, Defender, Astrosmash, etc.
I don't know how they are doing the emulators but it is flawless. I am really impressed.