Check this shit out: http://game-oldies.com/
There are other browser based emulators out there; this one seems to be the most well-rounded, flexible, and stable. The game controls can also be re-mapped. Using XPadder and a XBOX 360 controller works for me, and is almost mandatory IMO.
Some of my favorites:
Haunted Castle, the first of the Castlevania family to make an arcade appearance, is one I have never played prior. The site is full of games that were never released in the U.S. This could waste much of my spare time.