This was the second most popular idea on the IdeaTorrent:
This patch allows you to define two custom delimiters, one "left" and one "right", so that Ctrl+Doubleclick selects all text between the delimiters. By default I've set the left delimiter to '(' and the right to ')', but obviously there are many useful choices (curly braces for C programmers, brackets for HTML, etc).
The delimiters can be set by selecting a character and using Settings -> Delimiter selection -> Set left/right.
The flag "entire document" (in the same menu) controls whether or not the program makes the selection on the current line or if it looks for the delimiters in the entire document.