Don't know if this has been posted before, if so, sorry for the repeat.
This is a pretty cool site. I have most of these games via emulators at home but this is a good way to kill some time at work if things are slow. They have a bunch of games for NES, Arcade Classics, etc that you can play via emulator through a web browser.
http://www.everyvideogame.com/
Nothing like a little Tetris during lunchtime. Also, they have the old NES version of Sid Meier's Pirates, which is a personal favorite of mine.