Classic Atari Games Online
Microsoft and Atari got together to offer quite a few classic arcade games - Pong, Centipede, etc - playable online for free in your browser (preferably Internet Explorer, the site says). The Atari 40th Anniversary games are also playable on touch devices, but that's not required. Some looked more or less like the original Arcade versions, while some were spruced up with different graphics.
I tried a few using Firefox 15 and Opera 12 running on Linux, and they seemed to work fine. Google Chrome (actually Chromium 18) looked really wonky, for some reason.