1. Hex Fiend

2. Hex Editor

3. 0xED: http://www.suavetech.com/0xed/0xed.html

0xED is a native OS X hex editor based on the Cocoa framework.

Fast editing of large files.
Unlimited file size (limited by what the actual file system supports).
Small memory footprint.
Instant opening of files of any size.
Supports all normal text editor keyboard shortcuts.
HFS+ compatible.
Resource fork editing.
Full hex/text search/replace. 
Binary/text Cut/copy/paste support.
Main window is sizable.
Plug-in system to display your custom data types.
Requires Mac OS X 10.3.9 or newer.
Universal binary.
Free of char