:Sex will open a file explorer in a split window. I bet you'll remember that one. is.gd/4xZW
Use "_dd to delete one line without overriding the buffer. "_ is black hole register. "When writing to this register, nothing happens."
:w !sudo tee % will use sudo to write the open file (have you ever forgotten to `sudo vim /path/to/file`?) (via @gbretou)
CTRL-w K will switch vertical split into horizontal, CTRL-w H will switch a horizontal into a vertical. (via @mpetrsson)
From a command-line, vim scp://username@host//path/to/file to edit a remote file locally. (See is.gd/9dwq for dependencies.)