HxD is a very powerful hex editor with a host of unusual features.
The program can open files of any size, for instance. (There is a theoretical limit, but as it's 8 billion gigabytes you're most unlikely to notice.)
HxD opens even huge files very quickly. You can page up and down without any noticeable lag, even on underpowered hardware. A Find dialog helps locate information you need, like a text string, or you can use the global Replace dialog to find and replace specific strings, hex values, integer or floating point numbers.
HxD doesn't just stop at files, though. You're also able to open, view and edit your system memory, as well as carrying out raw editing of your hard drives, USB flash drives or other storage devices. Although you'd better be careful: a mistake with any of this could cause you serious problems.
And an extensive list on bonus features includes a checksum generator, the option to export data in various formats, tools to split, concatenate, securely delete and compare files, and more.