Groovy Hex Editor is a tool for editing binary formats. It's a fully functional hex editor with many advanced features. You can toggle the display between hex and decimal, and there is a built-in inspector that easily lets you see the hex, decimal, and binary equivalents of a group of bytes. Bookmark feature allows you to place bookmarks to various offsets in the file that you're working on, so that you can return to important file sections.
String sifter wizard makes finding text strings embedded in binary files easy. Change log with undo/redo and change highlighting make it easy to keep track of what you've modified. Version 1.6 may include unspecified updates, enhancements, or bug fixes.