XTiled by Michael C. Neel and Dylan Wolf, XNA library for loading and rendering TMX maps
XNA map loader by Kevin Gadd, extended by Stephen Belanger and Zach Musgrave
TiledSharp: Yet another C# TMX importer library, with Tiled 0.11 support. TiledSharp is a generic parser which can be used in any framework, but it cannot be used to render the maps. Available via NuGet.
TmxCSharp: Useful for multi-layer orthographic tile engines. No framework dependencies, used with a custom OpenTK tile engine soon to be open source, tested with Tiled 0.8.1 (multiple output formats). MIT license.
NTiled: Generic parser for 0.9.1 tiled maps. Available via NuGet.
tmx-mapper-pcl: PCL library for parsing Tiled map TMX files. This library could be used with MonoGame and Windows Runtime Universal apps.
bit.tiled Loads TMX file as objects. Aims to be fully compatible with native TMX files.
Diddy is an extensive framework for Monkey X that contains a module for loading and rendering TMX files. Supports orthogonal and isometric maps as both CSV and Base64 (uncompressed).