Currently, I have two data structures I developed to use in the engine. A bitmap and a directed graph.
A Bitmap for CPU sided texture access. It has a simple and easy to use API to interact with texture resources. You can access any pixel in the texture by querying the data at the pixel coordinates. I first used it to convert a equirectangular image to a cube map on the CPU, but in the future I intend to use it when reading back textures that were rendered to on the GPU.
A directed graph meant for use in the Frame Graph / Render Graph system. When I first started to implement a Render Graph for my Renderer, I was surprised to find that the C++ standard library did not contain an implementation for graphs data structures even though there is implementations for many other data structures. So I used my experience gained in my data structures class, to quickly put together a directed graph implementation specifically for the Render Graph. It is not intended to be a fully featured data structure but rather to support functionality for the Render Graph.