XVI32 is a hex editor for Windows developed by Christian Maas. The program is free, but it uses an unobtrusive nagware model. It has several nice features, but uses a poor UI, and, because it is a 32-bit program, it has some pretty strict limitations.
XVI32 was the first hex editor I used in Windows because it was free. However, after I kept running into file size limitations, I sought out a replacement and found Frhed, but, I eventually replaced that with HxD.