High-bypass turbofans, or just turbofans in general.

In the early days of jet airliners, you had high-velocity exhaust providing all of the thrust. With turbofans, you have much more thrust coming from low-velocity bypass. This is quieter in itself, but also 'shrouds' the high-velocity exhaust.

As bypass ratios get higher, engines generally get more efficient and quieter. The challenge in doing this is mostly engineering the large fans.