Vim delete blank lines

Found it, it's:

g/^\s*$/d

Source: Power of g at vim wikia

Brief explanation of :g

:[range]g/pattern/cmd

This acts on the specified [range] (default whole file), by executing the Ex command cmd for each line matching pattern (an Ex command is one starting with a colon such as :d for delete). Before executing cmd, "." is set to the current line.

:v/./d

or

:g/^$/d

or

:%!cat -s

The following can be used to remove only multi blank lines (reduce them to a single blank line) and leaving single blank lines intact:

:g/^\_$\n\_^$/d

:g/^$/d

:g will execute a command on lines which match a regex. The regex is 'blank line' and the command is :d (delete)