A lot of game systems, once you click the Play Game link on the game page, will have in the top-left corner of the play window "click here to play using Mednafen". Any game system which has that, can go fullscreen using the Mednafen emulator by tapping the Alt and Enter keys (this also applies to PlayStation games since the RGR Plugin is built on Mednafen for PS1 emulation).

Additionally, any game system which is supported by MESS can go fullscreen using the MESS link. Also, Arcade games now have fullscreen support with the latest RGR Plugin update.

Here's something you might not know: Nintendo 64 can also go fullscreen with the Alt and Enter keys, but only with the 1964 emulator. Depending on your graphic card, you might have to switch your monitor's color bit depth to 16 bit for Nintendo 64 fullscreen (this might not be supported in Windows 8 and above); you might also have to first click on the game screen before tapping the Alt & Enter keys. Instructions for changing the color bit depth can be found here:
https://www.vizzed.com/boards/thread.php?id=47212

Lastly, Super Nintendo can go fullscreen regardless if you are using Mednafen or not (although Mednafen may be your only option if your graphic card is incompatible with the fullscreen setting of ZSNES). Enjoy!

I re-opened this thread just so that I can promote my last edit regarding full-screen for n64.