The classic was the Portal Corp mission 'wolf farm' on the instance map that was the NE corner of Bricktown; the mission goal was to close the eight dimensional portals, but you ignored them. You needed a tanker (Inv worked best) and a /Dev blaster. The blaster flew to an open-top dumpster to the north on the east side and started putting down Trip Mines continuously while the tanker ran around aggroing the map (the trip mines would age out and vanish, so the blaster had to keep putting new ones downl. When the tanker had grabbed the map, they would call 'inbound', and the blaster flew up toward the height limit. The tanker would jump into the dumpster, and the wolves would jump in after them. Because mobs didn't have a vertical collision box, mobs could jump down onto each other and overlap, and you'd get about 90% of the map in the dumpster. Because of a quirk with Trip Mine, if the blaster that put it down was too far away, the mine wouldn't go off, so the 12—15 trip mines in the dumpster would just sit there. Once the tanker was sure they'd gotten all they could into the dumpster, they'd call 'in', and the blaster would dive down; when they got in range, all the mines would go off. All the wolves in the dumpster would go down, your Inspiration and Enhancement trays would instantly fill up, and you'd get around 600,000 to 800,000 XP. Exit, reset the mission, and repeat.
One of the funny things that would happen, if you were too close to the dumpster when the mines went off, is that you would get a pop up window from the game engine that said "Too many effects to render."
Unfortunately, the change to the aggro limit made it impossible to get the entire map at once, so the classic wolf farm fell to the game updates. Prior to that, though, putting limits on how many times you could reset a mission, and making that mission timed, restricted how much you could exploit it for XP and drops, but didn't stop it.
A lower- level open-zone version of this, with somewhat different tactics, came from before the changes to Burn that ended the Burn Tanker as the brain-dead leveling build, epitomized by the Burn Tanker -- and I'm sure someone still has screenshots of this -- running through Perez Park aggroing all the Skulls and Hellions on the streets and dragging them to the dumpster in the southeast corner of the zone, jumping in, and burning down all the mobs that followed them in.