In VIM, one of the features I always manage to forget exists is the g ex command.
Recently I had the need to delete all lines in a file containing a certain pattern. Using the g ex command, this sort of task is a breeze:
:g/pattern/d
This will delete all lines in your document containing pattern. You can then extend on this by using a regular expression as the pattern:
:g/^\[DEBUG\]/d
Which will result in all lines starting with [DEBUG] being deleted. This can be very useful for clearing up log files for simple analysis.
For a more in-depth look at the g ex command checkout the VIM Wiki article.