Ex-Mode let's you do all sorts of extremely useful things like running sed in-place on the current file, or only on the current selection text in the file, or d

Ex-Mode let's you do all sorts of extremely useful things like running sed in-place on the current file, or only on the current selection text in the file, or do something like sed but interactive that will do a regex search-and-replace on the current file/selection but at each match give you the option of skipping that one. Ex-Mode also lets you do a lot of things unrelated to sed, including running an arbitrary shell command and inserting the stdout into the file at the cursor, but all the sed-like commands are what blew me away.