Cmd + d selects the current word. Subsequent Cmd + d presses will select the following instance of the word for editing. Makes it easy to do things such as renaming a local variable or changing both the opening and closing element of a HTML tag.
What's really wild is if you do CMD+d several times and then move around with the arrow keys. Multiple cursors follow you around. I'm not sure if this is useful, but it's certainly cool.
Also good to note: Command-F find also has a "Find All" option which can be faster than pressing Command-D multiple times, and also allows more complex non-word and regexp searches. From there you can use multiple cursors or replace the whole selection just by typing.
Brilliant editor, this one.