Note: Right now this is more of a proof of concept than a fully working emulator. There are still some features that are not even emulated, such as the SID chip (for sound), and some of the graphical modes in the VIC-II chip.
It is possible to execute rom files in .prg format. In order to do so, drop the file on the emulators canvas area above. However, due to the fact that every feature in the original hardware isn't implemented yet, not all roms will be executing flawlessly.
If you want to try out a game running through this emulator, check out SoS fantastic implementation of Flappy birds for the C64. It can be found here.