Of course hard to compete with the bicycle lift.. but the most used invention is probably the Global Systems for Mobile communications, better known as GSM. GSM was invented by Torleiv Maseng and Odd Trandem here in Trondheim in 1980s.
Another candidate for the biggest Trondheim invention is the microscopic monosized plastic beads used e.g. for cleansing of bone marrow (in the case of cancer). This was created by Professor John Ugelstad in 1979 (at that time it was only possible to create such beads in space under weightlessness conditions)