• Command-option-period to close the most recent unclosed tag
• Control-shift-comma to select the current word and turn it into a pair of opening and closing tags (eg. html -> <html></html>)
• Control-shift-W to insert an opening/closing tag pair after the cursor, with the name highlighted (and set up so that both will change as you edit them)
You can also set these up in Vim, but it's nice to have it already there when you sit down to edit something