It has been a long time since I needed one, and it turned out that I did no longer have one available (installed) on my current machine: a hex editor. But there is ample choice on the web. I tried two of them: first Hex Fiend, from ridiculous_fish. You wouldn’t tell from looking at the homepage of the site, but the author(s?) has written serious code – and explanations about them to boot. Hex Fiend works well, and can supposedly handle very big files. That could come in handy.
My second test concerned 0xED, a tool (the only one?) by Suavetech. It has a somewhat different user interface, probably because it is a bit older. It works quite well too. Like Hex Fiend, it displays selected bytes in different interpretations, but it has more of them. As an extra, you can even write your own plugins to display your selection – that might come in handy if your dealing with somewhat more exotic data than text or numbers.
For the moment, I’ll leave 0xED on my disk.