There are quite a few drop spots in town where you can donate dry and canned cat food, including the APA building at 2807 Manchaca.
There are groups of volunteers in town who manage cat colonies by trapping, spaying/neutering, then returning feral cats back outside. By feeding these cats, they can also cut down on nuisance behavior that could arise.
The goal is to eventually get enough of these feral cats sterilized so we can reduce their numbers and stop killing them in our city shelter.
Mike Fry, from Animal Ark Shelter, did a great presentation on the management of feral cats at the No Kill Workshop last month. You can visit their website to get more information. Local groups, SpayAustin Coalition and Shadow Cats, also have great information on their websites.