The court in Colegiales (Parque Deportivo Colegiales) is at the 100 block of Freire. You can see it on the map, between Matienzo and Concepción Arenal. My boyfriend says that Saturday afternoons they have pickup games, around 2:30 pm or later.
For those who dare to venture out and can actually play ball, there are also games Saturday afternoons in Parque Chacabuco (you can take the E line there or the bus). The park is on Av. Curapaligüe between Asamblea and Eva Perón. The entrance might be on Emilio Mitre, but we are not completely sure. The guys are not as ¨civilized¨ as the folks at the Colegiales court, according to my boyfriend (think White Men Can´t Jump). Sounds more fun than Colegiales to me.
These are both outdoor courts.
Hope that helps.