Tangoblivion's milongas in Googlemap has a nice list of milonga locations mapped. Once BA streets are labeled by Google, this site will be more useful, but last time I checked, only a satellite view was available. Learn more about the project here.
FLOF (below-may be slow loading) is a similar Argentine site that already has streets mapped, but it requires free registration for best results, and it doesn't have many milongas marked (yet). Grab and drag to move around the map, or register at FLOF.