I've learned about the Arduino, and bought one for myself. Just last night, I learned that an LED can be used as a photodiode. http://news.ycombinator.com/item?id=447770

I'm also participating in "HN Reads SICP", a SICP book group that started on HN: http://news.ycombinator.com/item?id=428248

I've also attended three events that I probably would not have heard of without HN.