Trim Stab,
It's called "FLARM" and an increasing proportion of the UK glider fleet is thus equipped. It has its limitations, but the kit is definitely an aid to flight safety.
However, if can be a distraction when you are tugging at a competition and there are 50+ gliders in the vicinity of the airfield.
Like any bit of kit, if you use it sensibly and don't become a slave to its indications, it can make you safer.