The Open Terraria API is a cross platform Open Source Terraria API. It is compatible with both the Client and Server binaries, allowing you to build simple or complex mods.
Initially the API was forged using Mono.Cecil for the new TDSM Rebind Server Software but quickly outgrew and started supporting other Server Software like TShock 5 (Orion). OTAPI allows for very quick updates to get everyone back on their feet as quick as possible.
Recently the developers at OTAPI have started building on a new Client API so mods can add custom functionalities such as NPCs, Items and Shops.
Here is a very short list of the features OTAPI can provide: