File header inspector utility

I sometimes need/want to view raw data in certain file formats and I suddenly realised what i need is a program like a hex viewer but that can decode a bunch of different file formats (general ones like bmp, mp3, etc) and allow you to view and modify fields in the header of those files (as well as the actual file data).
I looked here and googled for it but i don't really know if such a program even exists so i was unsure of what to look for. I now vaguely remember using a hex editor which could do that but i don't think it supported many formats. And it wasn't free, i think it was UltraEdit or similar. I want a free one if possible and it must be able to support quite a lot of formats and maybe able to define new formats. Ok, i'm probably being a bit specific now, if anyone knows of any such program that would be great.
thanks.

 

http://www.techsupportalert.com/best-free-hex-editor.htm#hxd

chart to compare and note the latest release
http://en.wikipedia.org/wiki/Comparison_of_hex_editors

 

Thanks plodr. I have tried most of those hex editors before and although they were good i never found one i really liked. But HexEdit does the job and is generally a good hex editor so i think i'll stick with that.