we had a great experience with the salvation army. we called them up, set up a time for them to come pick it up, they came (on time!!), and even gave us a receipt for what we had donated. Phone number - 4911-7585.
If there is a decent amount, the Centro de Donaciones Don Orione will also do pick-ups. If not, you can bring them to the Red Cross, which is nearby in Colegiales (Cordoba 6434)...It's the Villa Crespo branch, but its not actually in V Crespo!
Alternatively, I think Un techo para mi pais, is still collecting clothes for those people in the BA provinces affected by the recent flooding.