http://osflash.org/fc64/

This, according to the link, is a full-fledged open-source flash-made Commodore 64 Emulator. Seems pretty neat.

Sadly I never had one of those, so could someone enlighten me how you're supposed to use a C64?

Holy wow!

Here, for a more authentic experience, type the following into the C64 window:

10 FOR C=0 TO 15
20 POKE 53280,C
30 NEXT C
40 GOTO 10
RUN

Then watch for 10 minutes before loading one of the game to the right ;)

Lmao. Nice one, Liam! :D