Obv if you were to include other in-game memes, you'd want to mention "Hey how do you..." "ctrl + k" or the old "(message) for admin" routine.
In terms of forums memes, I really liked when bandwagon threads were common or whatever degree of avatar fads - forced or otherwise. It was cool to see people contributing to one another's ideas.
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Oh! And cytube was a fun one too, where you'd watch YouTube videos together. Pretty sure Gatysh made that one a pretty eventful thing way back when