I know how you feel, it breaks my heart too. all you can do is volunteer whenever you have time, donate whatever you can when you can, but another thing is to do what you can to "advertise" (is that the right word, my mind is going blank right now) the subject of spay/neuter, it is really an important matter, it would cut down on the number of strays.
you didn't cause the problem so don't feel too bad, I have to remember that for me too, I get so down sometimes when I see those poor caged cats. its because of people that are too careless to get them fixed.

czicat-- thats so wonderful of you to do that!!