HxD Hex Editor
Full user review
"Clean, simple hex editor. F/F!"
Customizable formatting makes for easily distinguishable offsets and modified data. I use it for rom hacking. Fits right in with the rest of my tools.
Nothing, it's great!
I like the feel of the interface. I doubt I will get around to using the Analysis tools all that much, but I learned hex by using HxD and it made it a lot easier to get used to.