Tamriel Rebuilt (commonly abbreviated as TR) is the largest collaborative fan-based modding project for the Elder Scrolls series. It is working to create the mainland of Morrowind in Morrowind's engine. The project has existed since before the game's release, though the leadership and members have changed many times since then.
Earlier in Tamriel Rebuilt's development, there were plans to create Hammerfell in Oblivion's engine, but the only public release was the island of Stirk, west of Anvil. For various reasons this effort was abandoned and attention was returned entirely to Morrowind, where releases continue to this day expanding west and south. Project Tamriel, a group of projects in collaboration with Tamriel Rebuilt, aims to complete much of the rest of Tamriel, starting with Skyrim and Cyrodiil. Notably, its Province: Cyrodiil project has started with Stirk, recreating it on a larger scale than the old Oblivion mod.