Open Tibia Binary Map - a binary format in which game map is saved. It can only be viewed and edited using a special map editor, such as RME.
Remere's Map Editor - a map editor that can handle OTBM maps. Official site: https://remeresmapeditor.com
The Forgotten Server - engine for running your own OTS. It was originally a fork of OTServ. Now the most stable, powerful and up to date engine still in development. See: https://github.com/otland/forgottenserver.