Vector3 and friends, but backed by doubles instead of floats.
This free library gives you new data types in Unity. They provide the same functionality as the Unity maths and physics libraries, but they are built using double precision floating-point numbers.
The classes are designed to be instantly familiar to Unity users. There are very few caveats or missing methods. The included classes and methods are fully listed with comprehensive documentation here.
Consider of course that doubling the precision will affect memory usage and performance. If there's something you think is missing, feel free to get in touch.
Robbo