Widening can happen subjectively with the right EQ and dynamics processing, but specific widening processors are used in high-end mastering far less than anybody might believe, given what you read on the internet and in magazine advertisements.
When it does happen, most often it is M/S (Mid/Side) processing, often EQ, but sometimes dynamics, and sometimes simple M/S level adjustments (bringing up the "S" channel). It also tends to be more often digital than analog, though analog M/S is on the rise with several products offering the feature over the past few years.
The Weiss digital EQ is one common place to do M/S processing in mastering (but by no means only). You can put it into M/S mode and, for example, on a rock project you may simply EQ a couple dB up on the sides in the mids to upper-mids to make the guitars come alive, sounding huge and wide. You don't necessarily need to globally increase the side component.
Of course, you must be careful not to lose the center, or lose the sharp imaging as you start mucking with M/S. Other times, M/S is simply a corrective tool rather than a widening tool, as in M/S de-essing, or EQ on the "M" to fix a vocal.