I'm an OP on my friends' server and I'm wondering how to make NPCs. Apparently no mods are required.
You are unable to add NPCs to a server without adding mods. However, some popular Bukkit plugins that allow you to add NPCs include:
Implements everquest style npc spawngroups using the npc library allowing you to make persistent npcs/rare npcs spawn around your map. Npcs will fight back if attacked and even talk on right click. They will sell items and can even have loot assigned and be given factions and paths to walk
The point of NpcSpirit is to create an Npc system that will tie in well with world economies and player constructed environments, rather than having them be an administrated constant. A certain kind of block (sponge by default but editable in config) is denoted as containing the souls of NPCs. When this is placed, it spawns an npc and will continue to do so when the npc is killed. Admins or the Npc's owner may change the npc's name(and associated skin pulled from minecraft.net), equipped item, greetings, threats and conversation.
Citizens is an NPC plugin in the vein of the old hey0 ones, aimed at creating NPCs that perform a variety of tasks, such as dispensing information. Currently NPCs can be created and say a set line once rightclicked with a specified item, however, more features will be added soon, such as traders and more. Citizens supports Permissions but will default to ops only if not available.