This page lists the IDs of NPCs in Terraria. This includes monsters, critters, friendly NPCs (otherwise just referred to as 'NPCs'), bosses, and creature parts if the creature is composed by more than one.
NOTE: That all NPCs have IDs, however, newer NPCs and variations of certain types use negative numbers as their identifiers. They're listed here for completeness' sake. ID#'s 1 - 147 v1.1 Items, ID#'s 148 - 296 v1.2 Items.