For gliders, FLARM is a radio-based system that also takes turning rates into account. It alerts the pilot visually and audibly 20 seconds before a possible impact would happen. In my experience it works great even in very congested airspace such as when circling in a thermal. it's not perfect, but it's a very good complement to looking out the window.

It only tells you about other FLARM-using aircraft, though. (Some also look for transponder-carrying aircraft).