In the Crop tool, Aspect text field default to 1:1 in gimp 2.3.14.
It would be great to have a combo box with current layer ratio, and some other common ones.
If current layer is 4000x2500 pixels, it should show:
> 4000:2500      
> 1:1 
> 1:2 
> 2:3   
> 4:3
> 16:9
> ....

first item should be editable as 1:1 currently is.
other items should be common ones + last used.





Other information:
It may be useful to have reduced forms of ratio, like 4:3 for 640:480.

Euclidian algorithm is useful to rewrite ratios, see http://en.wikipedia.org/wiki/Euclidean_algorithm .

Here is a python example:

>>> def gcd(a,b):
...     while b!=0:
...             t=b
...             b=a % b
...             a=t
...     return a
... 
>>> def pfrac(a,b):
...     d=gcd(a,b)
...     print a/d,b/d
...
>>> pfrac(80,45)
16 9
>>> pfrac(640,400)
8 5
>>> pfrac(640,480)
4 3
>>> pfrac(3008,2008)
376 251