These are obtained by getting high scores in games. It is usually easiest to obtain one after Score Reset Night.
Some games have a limit to how many points can be scored and you get a gold trophy simply if you can match that perfect score. Some non-flash games such as Cheat! give you trophies if you simply beat levels or rounds rather than beating other players.
Site Event Trophies
These are trophies that are given out for participating in certain events and therefore they are all technically retired.