Hex editors for Windows?

Free Hex Editor (frhed), small and fast.

Personally, I use HxD (Free but not open source). Particular features of note include the ability to open very very large files, and you can modify raw disk data and open and edit data in RAM.

XVI32 is a freeware hex editor running under Windows 95, Windows 98, Windows NT, Windows 2000, and Windows XP.

Cygnus Hex Editor is a powerful file editor for Microsoft Windows. Unlike text editors, which are designed to edit files that contain text data, hex editors allow you to edit files that contain any type of data.

Free Hex Editor Neo is award-winning large files optimized freeware editor for everyone who works with ASCII, hex, decimal, float, double and binary data.

HxD is a carefully designed and fast hex editor which, additionally to raw disk editing and modifying of main memory (RAM), handles files of any size.