I don't think I've seen this feature on CO's web site before, unless I'm getting senile in my young age.

It used to be that you could only get the seat map for selecting a seat when you make a new reservation.

Now, when you view your itineraries for existing tickets/reservations, there is an option to "View / Change Seat(s)". This will bring up the seat map of the flights you're on. No need to call the 1-800 number anymore to change seat assignments.

You can also see if the seat next to you is unassigned, and get an idea of how full the flight is.