I've been hooked. What can I do?

The following are all things that I've seen work for me and my teammates, playing from early in the closed beta to now.

Reinhardt's shield and Winston's bubble both block the hook, so if they can be raised before the hook hits anyone, that can save not only the player, but also their teammates.

As one of the other answers has said, using a character that has an "out," like a dash, you can semi-reliably escape from the Roadhog.

Dashing with Tracer or Genji, or using Tracer's Rewind, might save you if the Roadhog isn't fast enough. Likewise with Pharah's Jump Jet. McCree can try and stun the Roadhog and roll away, which might work in his favor.

Reaper and Mei can easily use their invulnerability abilities to help prevent their demise. Unfortunately, Mei's leaves her right there next to the Roadhog, so you have to hope your team can save you. Zarya's barriers both block the hook, if applied before it lands. Also, Zarya can activate a barrier on herself or an ally after they've been hooked to hopefully ensure the Roadhog does not get a kill, and instead gives her a big boost of energy. Call out to a Zarya on your team if you have one, and they might keep you alive.

Winston and Zenyatta can pop their Ultimate to pretty much ensure they survive the encounter, but that's probably a wasted ult.

Lastly, one thing that works for me about half the time, since I play Lucio very frequently, is to use a knockback ability (Lucio's alt-fire) on the Roadhog, which sometimes prevents a killing blow from landing and occasionally disorients the Roadhog. This can also work for Junkrat and his concussive mine. Press shift and right-click immediately. The Junkrat will be blown back, and the Roadhog might even die. If Pharah's Jump Jet is on cooldown, her Concussive Blast would knock both characters away from each other, and might be more effective than Lucio's alt-fire.

One important note with using knockback abilities: Junkrat's and Pharah's abilities move both players, so they usually work a little better in certain situations. If the Roadhog hooks you as Lucio while standing backed up against a wall, trying to blast him away with your alt-fire will be much less effective, if not pointless.

Something that could work against a very slow or just bad Roadhog is Hanzo's scatter shot, fired at the Roadhog's/Hanzo's feet. I've never seen this successfully done, but theoretically it might work.

One final note: Widowmaker's grapple probably wouldn't work, as it is very slow, but poisoning the Roadhog with a venom mine as you die might ensure that he goes down, too.

Normally, you will die. Except if...

Factors outside of your control:

• Roadhog's competence. One that is completely new at the game might not shoot you in time, giving you enough leeway to use a defensive ability.
• Roadhog's lag.
• An ally saved you: you have an armor pack, zarya bubble, a mei wall, another hook, etc disrupting the roadhog combo.

Factor partially in your control:

• There is a bug with fast moving characters that will sometimes have you hooked behind or to the side of the Roadhog. If you are genji, tracer, or lucio, this seems to happen more often if you are moving around very quickly.

There is no skill that makes you escape from a Roadhog hook. He can shoot mid-hook, and therefore will hit you with his shotgun while you are still stunned. Some Roadhogs don't utilize this, and one tip is to hold the "escape" (Mei Cryo-Freeze, Tracer's Recall, etc.) button down, instead of smashing it 10 times a second. Like this, the skill will activate as soon as it can be activated.

You can deal with Roadhog by staying at long range. Due to his big model, he is easy to hit, even at long range. If you can't stay at long range, stay behind your tanks. If you play a fast character like Tracer, you can kite him around or bait out the hook.

tl;dr: Avoid getting hooked. Once Roadhog hits you, you're dead.