So it's night time, and you want to take a picture of a friend, a self portrait, whatever, and there's a city in the background. But there's a problem, you take the picture, and it's lame, just a photo of the person with harsh lighting if you use a flash, and just a city and a blurry sillhouette if you dont use the flash.
Put your camera on manual, do a long exposure WITH FLASH, as soon as the flash fires, have your model move out of frame, and the model will be sharp, but semi transparent, as the city was taken into the photo in the subsequent 6 seconds or so.

Canon A570 IS
6 seconds
F5
ISO 80
Image Stabilization Off (used tripod)
Flash did fire. (medium intensity)
And model placement/composition is important, depending on if you want their face to have lights in them.
And if you want them to not be transparent, just have them hold still for the duration of the picture, while the background is exposed, and the photo will still be in focus.
Of course, the camera mounted flash is incredibly harsh looking on skin, so if you feel like it, experiment with car headlights, flashlights, strobes, and other external light sources/flashes, I'm gonna take a mag light next time, and use some tissue paper to soften the light.