CMD+SHIFT+L - Breaks a multi-line selection into multiple selections, one per line.
ESC - Goes from using multiple cursors back to one cursor.
ALT+Drag - Inserts a new cursor on each line that is touched during the drag operation.
CMD+Click - Inserts a new cursor at each click location.
CMD+U - Undoes a cursor or selection change.
CMD+SHIFT+U - Redoes a cursor or selection change.
CMD+Drag - Adds a new selection and cursor.
CMD+SHIFT+Drag - Removes a selection and cursor.
CMD+D - Adds the next match as an additional selection and cursor.
CMD+K,CMD+D - Removes the current match from the selection and adds the next match as an additional selection and cursor.
CTRL+CMD+G - Adds every match as a new selection and cursor.
CTRL+SHIFT+Up/Down - Adds a new cursor on the next/following line, respectively.
TAB / SHIFT+TAB - Indents and dedents a multi-line selection, respectively.
CMD+] / CMD+[ - Indents and dedents each individual line that has a cursor.
Find All - This isn't a "key command"; but, the Find All feature will allow you to add a new selection and cursor to every matching piece of text. This is particularly powerful when combined with Regular Expressions.
Copy/Paste - Multiple selections can be copied and pasted at individual cursors.