UnrealMeshProcessingTools
Sample code for Mesh/Geometry Processing in Unreal Engine.
geometry3Sharp
A pure C# library for geometric computations like 2D/3D Vector Math, Curves and Surfaces, Spatial Queries, and Implicit Surfaces. A powerful adaptive Triangle Mesh class, and many mesh processing algorithms, like Remeshing and Simplification. Boost License, completely free for commercial use. See the Github Readme for more details. Samples and Demos here. A Nuget package is also available.
gsSlicer
A C# library for converting 3D triangle meshes into GCode toolpaths suitable for 3D printing, on your Makerbot, RepRap, or what-have-you. Under active development, but already quite functional. Excellent handling of overlapping shells and open-sheet meshes. MIT License. See the gsSlicerApps repository for demos and a 2D slicer/viewer.
gsGCode
A C# library for GCode I/O. Generate GCode from 2D and 3D polyline/polygon paths, or parse an existing GCode file back into those paths, change them, and write it back out. If you want to start with a mesh, use gsSlicer. If you want to directly control your 3D printer, gsGCode will handle all the messy stuff for you. MIT License.