I needed a quick and dirty way to combine a bunch of equal-sized images into a sprite sheet. I didn't find a third-party solution that worked out of the box so I just wrote my own.
It's a C# Windows Console app that takes in a path mask to where all your images are and where you want your output sprite sheet. Then it does the rest.
Download link: https://github.com/Gamieon/combineimagesastiles