I know I've been guilty of using the "Bar Brawl" or the "Adventurer's Noticeboard" cliche's every once in a while but here are a few of the other ways I've brought a party of PCs together:
"It's a lovely summers day and the grass is even tastier than it was yesterday."
This was the opening line in a Justifiers RPG scenario where the PCs were all beta-humanoid sheep. They had been on a mission, to investigate the dissappearance of a fellow Justifier team, which had gone horribly wrong and they began the adventure in a pasture having been subjected to a neural wipe without any memory of themselves, their past or their species. Over a couple of sessions they gradually discovered their identities and skills as the neural wipe wore off.
"The guy in the unseasonal outfit seems to have wires running from his sleeve to something in his hand..."
In this Squadron UK scenario (see Project Daedalus postings) the PCs are the only survivors of a terrorist dirty bomb in Trafalgar Square. The bomb awakens their latent superpowers (the genetic legacy of their grandparents) and during their recuperation in a secret government facility they get to know one another and explore their new found powers.
"You notice that the ship's motley crew are carrying short lengths of chain with manacles at each end. Something tells you that you should start running..."
I started one of my The Lands of Dual fantasy campaigns by collecting together each of the players from their respective homelands with a slave pirate ship. It's amazing how quickly a party bond with each other when they're chained together rowing for 18 hours a day!!
"You best be getting up to the Castle before the Siege starts..."
In this campaign opener the PCs were press ganged into becoming siege defenders by the Baron of a coastal town. The PCs were unaware that the Baron was being controlled by the forces of darkness and that the King's forces, massing on the hills outside of town, were being led by a particularly devout Warrior Priest with strict instructions to erase the castle and it's inhabitants from history.