This is a favorite video of mine showing Joseph using ZBrush to create a low poly game mesh from a high poly mesh. I really like it because the whole process and workflow of creating low-poly game objects with ZBrush is shown here: Retopology of a high-poly model, UV-Mapping of the low-poly model, projecting details of a high-resolution model to a low-resolution model and finally baking normalmaps and materials to textures.
In part one of this ZBrush tutorial he is using ZSphere to retopo the high resoultion model:
In the second part he is slicing the model into polygroups to create a good UV-mapping with ZBrush and the plugin UV-Master:
In the last part is explained how to create maps like a nornmalmap for the game-object and also how to bake the selected material into a texture by using the MatCap-plugin:
There is a lot to learn following the steps of this ZBrush tutorial when you want to create nice looking and effective low poly game meshes.