Here are a few of the ones I have used:
- You all grew up in the same village and have known each other since childhood; some of the PCs are related to each other by blood.
When I used that, we did some random roles to decide everyone's relationship with each other. Two of the characters were half-brothers (one was a half-elf!) and two others were grandfather and granddaughter, the rest grew up in the same village or nearby and had known each other for many years. Since all the characters were related, it did cut down on the "how do you know each other?" stuff, and for fun a couple of the characters even made up some "stories from the old days" to create a little history between their characters.
- All the characters feel themselves drawn to a particular spot in the woods along a trail one night; a gypsy witch appears and tells them all they have a shared destiny and must stay together to accomplish it, she then disappears leaving behind a curio (a key, a necklace, a ring, etc).
The DM can decide what the mysterious "destiny" is as the campaign goes along, what part the curio plays in the destiny, or to make it more fun let the characters figure it out themselves through actual game play. It can be a complete McGuffin and never really turn out to be anything, or the DM can spring it on them when they get to high level and are ready to face a deadly or powerful foe ("You get a feeling, hearing about the Lich King of Daktoma, that this is the destiny that was spoken of many years ago, and the amulet the mysterious witch left for you has some kind of connection to the Lich King")
- The characters are all in one spot and a attack is made there; they must flee with the other PCs and form a group to survive the escape.
I used this to good effect in one campaign; the characters were all in a tavern when it was attacked by frost giants and white dragons, who were destroying the town for not paying the annual tribute to the giants. The characters all managed to escape into the woods, and had to travel across a mountain range and forest to reach a major city and let them know. This caused them to quickly bond with the shared goal of survival, and by the time they reached the city they were 3-4th level and had formed a tight group, sworn to revenge themselves on the giants.
- The characters, through one way or another, all work for the same guild or merchant group.
This also worked well with a group of mine. All the characters were in the employ of House Valaryan, a merchant house with an outfit in a jungle outpost. Some of them were related to the upper members of the merchant house; some were merely paid employees; others owed a debt to the House that they were paying off through indentured servitude. They often got sent on "missions" together so they bonded well through the shared experience of having the same employer.
- All the characters are being stalked by the same enemy, and must band together for protection.
Through various methods, the characters have all run afoul of an evil organization, monster or individual and are being hunted down and killed. They bond together for mutual protection, plus they can only let people like themselves know who they really are. The foes could be a evil religion that is hunting down unbelievers (any religious beliefs not worshipping the evil god, which happens to be all the PCs); a lich is sending out servants to find his lost phyactery which he believes one of the characters holds (none of them do, but the lich isn't very reasonable, and puts them all on his death list); or a powerful lord or King is hunting down members of the family who originally deposed him...now back in power, his foes are long dead, but he will revenge himself on their children and relatives.