GIANT SNAKE: Unless I'm missing something, this might be the first proper appearance for giant snakes. They're mentioned here and there, and a sea-dwelling variety is introduced in Supplement II: Blackmoor. They do show up in the ubiquitous Wandering Monster tables of Supplement III. But this is, so far as I can tell, the first entry for your average, land-dwelling Snake of Unusual Size. Huh.
As Gary has seen fit to provide us with a good variety of snakes, I'm going to treat them as I would a new monster, and outline each one below.
Amphisbaena: This thing is just delightfully weird: a snake with a head at each end that moves around by forming a circle and rolling around like a hoop. This is the sort of thing that's just too wacky to spring from Gary's mind, and sure enough I see that it's a genuine mythological creature. The D&D version is a deadly customer: it has 6 hit dice, two attacks a round, and poison that can kill instantly on a failed save. There's nothing sad here about the amphisbaena's origin, but I'm willing to work with the version from Greek mythology, which states that they were born from the blood that dripped from Medusa's head as Perseus flew with it over the desert. That's pretty rad.
Constrictor: As you might have guessed, this variety of snake likes to drop down on its prey and squeeze them to death. There are guidelines given for characters who want to pry the snake from its victim (you'll need four humans each with a Strength of 16+), but nothing said about what the victim can do to escape if he's alone. Get hopelessly crushed to death seems to be the answer, though I would err on the side of giving them a chance to escape (with a dagger, perhaps).
Poisonous: The standard variety venomous snake. It's not clear that said venom is lethal on a failed save, but I would assume so. There are some varieties that even inflict 3-18 damage on a successful save, and in AD&D that can be a hefty sum, even to a high-level PC.
Sea Snake: The Sea Snake is the closest to that which had stats in Supplement II, so let's do a comparison: AC has improved from 6 to 5; Movement was 20", but has slowed to 12"; Hit Dice was 6, but is now 8-10; bite damage remain at 1-6, constriction damage goes from 2-8 to 3-18. The snakes from Supplement II had the ability to swallow people whole, like a Purple Worm, but that's not present in the Monster Manual. They still like to coil around small ships and crush them, however. And now, to justify these changes! I figure that this is a larger variety of sea snake we're seeing now, which makes it slower, but with thicker scales. As for the loss of the swallow attack, well... I'm stumped. Perhaps they're just that little bit slower, and can't strike fast enough to surprise someone and swallow them. That'll do.
Spitting: Spitting snakes are exactly like the poisonous variety above, only they can spit their poison up to 3". Nasty.