emulation is obviously the best way to get these games.
but i get what you mean.. there are a lot of freeware and shareware games that are based on classic arcade games.
this is a fun 'based on' galaga game
http://www.warblade.as/index.asp
there was an amiga version originally, years ago by the same guy "deluxe galaga" it was a lot of fun.
i love aliens arcade games.
aliens, alien 3; the gun, alien versus predator