Paste a list of places. Get the distance from every one to every other, in a grid. Free, no signup.
Straight-line (great-circle) distance, which is the standard for matrix comparisons, territory work, and site selection shortlists. For a drivable number between two specific places, use the drive time calculator.
12 places is 66 pairs, which keeps the grid readable. Need more? The InstaMaps add-on computes distances for whole spreadsheet columns.