PCAS for traffic spotting
I know it is by no means 100%, but many times I have been flying along, looking out to the best of my abilities - and also many times, head down looking at maps etc. - when my little portable PCAS unit goes off and warns me of another aircraft.
Just a few weeks ago I was flying along maybe 500-1,000ft below a broken cloud base, and the unit went off - warned me of an aircraft ahead, same level. I had been looking out but just had not seen it. I quickly descended by a few hundred feet and very shortly after saw a twin only a few hundred yards away, pretty much nose-on to me, which passed a few hundred feet overhead. Based on the 'big sky' we probably would not have hit each other, but it would have been close.
The unit itself is pretty small, very light, sits easily on top of the panel, and uses 12V power from the panel (or a battery unit I carry) so is no problem to use in the club aircraft that I rent.
It doesn't necessarily give peace of mind, but at least it helps a lot to spot aircraft that are transponding Mode C (or S) and which are a 'threat'. (Of course not all aircraft have Mode C/S, like perhaps the smaller aircraft in the Coventry accident, but it tends to be most of the faster ones, where you would have less time to avoid.)
It was pretty expensive - over £1,200 - but having used a PCAS unit I would be pretty reluctant to fly without.