This is a tip for command-line users. Want to read a man page? Just click Terminal’s help menu, then type the first few letters of the command. The menu beneath will autocomplete the rest, along with other suggestions. Then click on the command whose man page you want to see and it’ll open in a special viewer.
Because of the autocompletion, this can be a very nice way of exploring OS X/Unix commands.
For what it’s worth, another way of viewing all the OS X/Unix commands is to press and hold Esc within a Terminal window, then answer Y to displaying all possibilities. (Hit Q to quit the scrolling list.)