[vim] Simple Vim commands you wish you'd known earlier

Taking xcramps' suggestion one step further, I can't tell you how many times I've used:

:%!sort

to sort a list of items in a file.

Details:

:range!command

will execute a shell command on the specified range of lines. A range is usually specified as start,end

Examples:
1,3 specifies the first 3 lines
'a,'b selects the text between bookmarks a and b
.,$ selects the entire document (. = first line; $ = last line)
% is a shortcut for .,$ and also selects the entire document.

Feel free to mix and match numbers, bookmarks, ., and $.