RuneScape.

Some guy found an invincibility glitch and used it to attack a monster with a good drop table.
He created 30+ accounts to do this and created a macro whilst he was at it.

A 15 year old kid made much upwards of $50,000 in a month.

One of the moderators was recently fired due to coding a special program in-game just for him.
Many suspect that this was to spawn runescape money to sell in real life, as the prices of gold had dropped considerably.
He also posted a facebook picture of a nice new car, much nicer than any other Runescape/Jagex employees.