Hex Fiend's developer (who goes by the name of "ridiculous_fish") describes it as "a fast and clever free hex editor for Mac OS X." Hex editors tend to be fairly bare-bones affair, but Hex Fiend has some nice features, like the ability to insert, delete, and rearrange data, a data inspector that lets you view data as integers, floating point, signed or unsigned, big or little endian, and so on, and capacity for huge files. And it's free.