Implicit and Explicit Feedback — for preferences and recommendations, implicit signals (what people clicked on and actually listened to) turn out to be strongly correlated with what they would say if you asked. (via Greg Linden)
Twitter Storm (GitHub) — distributed realtime computation system, intended for realtime what Hadoop is to batch processing. Interesting because you improve most reporting and control systems when you move them closer to real-time. Eclipse-licensed open source.