I've had a few near collisions. The most alarming occured while I was flying a night search over our local lake, in my capacity as a firefighter. I had called the Armed Forces to assist, and they had dispatched a C 130, and Cormorant rescue helicopter. I had informed the C-130 pilot of my presence there, and altitude, and he had acknoledged.
While I orbited over our fireboat at 500 feet up, to allow the armed forces aircraft to use their more advanced search equipment, The helicopter flew directly under me, (I had lights on, he did not - later citing the need for darkness to use his night vision!) All I saw were his heat signatures agaist the reflected moonlight on the lake. I have no idea how close he came to me, but I never saw him.
He reported not having seen me, when I called to complain the next day. I reminded him that we were both in uncontrolled airspace, and he is required to display lights for night flying. This was apparently not their normal procedure! They also reported that they were not TCAS equipped - I found that very alarming too!