This one's a little old, but it's still a lot of fun to set up - particularly in a multiplayer game where you don't have to worry about cops swarming you the second you fire a shot. To set this up, find a gas station (there's one just north of the southern freeway in Broker), jack at least two cars and park them next to one of the gas pumps. Then, put a little bit of distance between you and the pump - about a car length should do - and line yourself up so that there's not anything immediately behind or above you.
Above: Oh, sure, this is fine for amateurs - but it's more fun if you can make it hurl you across town
Now, squeeze off a few shots at the gas pump - the ensuing explosion should catapult you into the air, soaring over any nearby buildings and coming down with a splat at least several blocks away. (At least, that's the distance we usually got with this trick - we've heard it's possible to be hurled to the other side of the map.)
What makes this especially fun is that it's survivable - it's actually possible to get back up and dust yourself off after you land, although you'll need a health pickup pretty badly. For the best results, try packing in a few additional cars near the pump, and stand on top of one of them.
We're not entirely sure if this counts as a glitch, either, but it's weird and unnatural, so in it goes. If you're driving a large, truck-like vehicle, like a bus, big rig or NOOSE wagon, it's possible to flip it onto its side spin the thing rapidly in a circle. (Three Stooges "woop woop" sound effects not included.)
To make your truck do a Curly Howard impression, you'll need to hit a ramp or like object with just one side of your wheels to tip the thing over. Then, once you're on your side, just hold the left thumbstick left or right and jam on the throttle. Assuming there's nothing in your way, you should start spinning, faster and faster, in a circle. This is great for taking out crowds of civilians, or just for making yourself completely unapproachable in multiplayer.
This is a weird one. By now, probably every GTA IV player has tried to car-surf at least once, only to be thrown off by the all-too-realistic physics. But if you try climbing onto the hood of a big rig - which is here defined as any truck large enough to have a grill you can hang off of and a hood you can stand on - the results are just a little more spectacular:
Once you've clambered onto the hood, hold still and wait for the angry driver to start moving. In seconds, you should be launched into the air, head over heels, and come crashing down on the pavement. Usually, we only get catapulted a few feet into the air, but it's possible to launch yourself 20 feet or so skyward if you time your clambering just right.