These are the only 3 I can think of at present

Amumu
Maplhite
Galio

Who are the other initiators?

I would say anybody built for tank. main ones are Alistar, rammus, shen, talon, taric, really anybody tbh if they are built for tank

ashe has a good one with her ult.
blitz with a good grab

any tank character seijuani, naut, j4

Anyone who can run into the middle of a team, soak up some hits, do some cc and give the damage dealers the chance to do some damage.

Anyone who can split a team up i find, Naut and blitz with their grabs, Alistar with his headbutt

But anyone can do it, just someone with hp who can take a few hits if needed to help split em.

Maokai.. how are you forgetting him? really underrated tank. Jungles at a decent speed and ganks like a boss. not to mention his passive procs on every hit during team fights due to all the ability spam.

Initiation doesn't have to be AoE.

Taric can initiate a teamfight for example, by catching someone out of position and stunning them.

AoE CC is strong and makes for great initiation for a teamfight, but it's not the only way to start a fight in your favor

Darius can initiate in late game too, if he can grab the squishy carry and bring him to the group.

Considering Darius' range on his grab, no, he is not an initiator. Here is a list:

Alistar
Amumu
Annie (ult+stun)
Ashe
Blitzcrank
Cassiopeia (with her ult if you are lucky, not reliable)
Diana (Late game if built tank)
Fizz
Galio
GP
Gragas
Hecarim
Jarvann
Kennen
Leona
Lulu (assuming your tank goes in, you can ult them for a knockup, requires coordination, should not be used consistently at lower ELOs, can also be used to counter an intiation like malphite or kennen)
Malphite
Maokai
Morgana (Hourglass+ult)
Nautilus (he has 2! Grab AND his ult)
Orianna (ult is hard to use for some)
Pantheon (he is flimsy sometimes, but if you position your ult well and your team is alert, it does work)
Poppy (if you can dig in and reach a high priority target, they will waste cooldowns on her)
Rammus
Sejuani
Shen (can taunt OR counter an initiation with his ult)
Shyvanna (iffy)
Singed (iffy but workeable with good coordination and sticky glue)
Skarner (ult the carry!)
Sona
Swain
Tristana (jump in, ult and flash out or jump out. It's hard to do but possible, not reliable)
Trundle
Twitch
Urgot
Varus
Veigar
Viktor
Vlad (his ult has no CC... so your team needs to be quick)
Volibear
Warwick
Wukong
Xerath
Xin Zhao
Yorrick (more of a counter intiator, can ult the target of the initiation)
Zilean (same as above. Not an initiator per se)
Zyra

Err, Morgana and her ulti?