PeekIt is a quick and easy utility for examining any file on your Mac.
PeekIt allows you to open, examine, and change (as raw or hex values) the contents of a file. This is very useful for anyone from programmers to people who just want to snoop around the system to learn more.
PeekIt also allows you to annotate files, adding notes about interesting data locations, fields, and other information. This can be a huge boon to programmers working in remote teams, and with the addition of exporting .pinotes files in 2.4 this gets even better, see the Notes section of the docs.