My hex library, designed and built for Echoes of Astertide. Like many other hex libraries, you can use it to convert a world or screen position into hex coordinates and back. SturdyHex is based in part on the algorithms in Amit Patel's fantastic hex grid guide, which I would strongly recommend to anyone who wants to know how to represent and manipulate hexes.
I've implemented several features beyond the ones in that guide; in particular, I've found the DCEL-based traversal features to be a life-saver for procedural generation. I'm not aware of any other hex libraries that support this kind of traversal, but if you know of any, let me know! Stay tuned for blog posts covering extra features and DCELs for procedural content.