There are the Alchemist, Brewer, Young Brewer, Jeweler.
The Alchemist spawns after defeating Eye Of Cthulhu.
The Brewer spawns after defeating Eye Of Cthulhu.
The Young Brewer spawns after entering Hardmode with the Alchemist and Brewer alive.
The Jeweler spawns after defeating Eye Of Cthulhu.
Alchemist is the main npc. He can sell Health potions, Mana potions, Teleportation potion after the start of hardmode, Fishing potion after completing 5 fishing quests. Also he can sell some Tremor materials and flasks, including some potions from imkSushi's mod.
Brewer sells vanilla and (MorePotions, Reduced Grinding, Thorium and Calamity) potions and also some from the AlchemistNPC mod.
Young Brewer sells Battle, Mage, Ranger, Summoner, Tank, Builder, Explorer, Fishing combinations.
Jeweler is moderately useful.He can sell gems such as Amethyst and Topaz.After defeating Eater of Worlds/Brain of Ctulhu he sells Sapphires, Emeralds and bands.After defeating Skeletron he sells Sturdy fossils, Ruby and Diamond..If you have Thorium Mod or Spirit Mod he sells Marble and Granite chunks.
He also has a arena shop. At the start he sells Campfires, after defeating Skeletron he sells the rest of useful arena placables.