wobble plots the third position variability as an indicator of a potential coding region.
Here is a sample session with wobble. The example sequence is from Pseudomonas aeruginosa, which has a high G+C content and a very biased third codon position. If it can be G or C, it usually is:
% wobble Wobble base plot Input sequence: embl:paamir Graph type [x11]: Output file [paamir.wobble]: