Facepunch doesn't provide their own map editor – but we have tools to make it (Rust.World SDK). At this moment there are two map editors available.
RustEdit is a standalone map editor (meaning it is a click-to-run executable) with a range of tools for terrain, splat, biome, topology, alpha and prefab editing. Simplicity and good amount of features makes it preferred for beginners and regular map makers. Keep in mind that some tools are limited and available only for Patreon members.
This editor has its own Oxide extension that implements some missing features from the Rust.World SDK through modding, allowing you to modify Cargo Ship/Bradley APC paths, create your own puzzles and modify/add custom vending machines and loot tables. It also fixes some minor issues with deployable objects, respawn timers of crates and collectable resources, etc.
You cannot modify or add any tools within the editor – so if you want to do something that is hard or impossible to do with RustEdit, you should consider trying out the editor below.
Rust Map Editor (formerly known as Rust Map Making Editor, aka RMME) is an open-source map editor for Unity Editor made by Adsito. It provides the same basic functions as RustEdit and gives much more opportunities for terrain and splat editing. It is recommended only for advanced map makers that have experience of working with Unity and C# (recommended but not necessary)
Since it is based on Unity Editor and open-source, you can import third party editing tools via packages or Asset Store, or code your own tools.
Information gathered provided by the Rust Map Making community.