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.
Best Answer
Well the "alternate fire" is just like his "primary fire" except with a longer range. It launches a ball of scrap, and after a short distance, it explodes into a normal shotgun blast. It's meant to be used to kill or harrass people who are outside of your regular primary fire range, assuming your hook isn't available at the time.
A similar question has already been answered on Reddit, where someone has posted a diagram portraying the use of Roadhog's "alternate fire", credit to Redditor _SpliceD;