I played EVE online as a pirate for years, I could name so many mean spirited things I've done to so many people I can't even begin to count, everything from organizing suicide ganking ops on expensive, cargo laiden freighters, to just generally stealing ore from people and blowing them up when they try to fight me... Also, well, all sorts of underhanded trickery and theft from corporations, spying, all sorts of "fun".
In other games though, well I tend to keep my griefing to a minimum, but every now and again, if it's possible I'll get people killed on my team in various ways, or train mobs onto people in various MMORPGs.
An example of this from way back, I was playing Deus Ex multiplayer, and I kept lighting a stealthy sniper team mate on fire, now, the fire didn't hurt him, but it lit him up for all to see, both on thermal scans, and straight visuals.
Still, lately, most of the mean spirited trickery I cause in games, are more just intentionally playing terribly in such a way that it doesn't hurt my score much, but ruins other people's team experience enough that they get angry at me and leave... Maybe I have social issues, or something, but really, it's not as if I harass anyone or follow someone to another server, or anything, mainly it's short bursts, and very convenience dependent griefing.
Yeah...
That...