Here's a tip I wanted to share about JOSM:

Oftentimes you want to make ways, areas, points, etc perfectly orthogonal or perpendicular. For instance, you want to grid-fit all streets in a "grid-type" city, or you want to align buildings perfectly to a street. Here's how to do it in JOSM:

Find a long, straight feature like a main street or a canal. This will be your baseline. Then draw a huge rectangle that easily covers the area of interest. Select the body of the rectangle and two endnodes of the baseline, and press Q. This will align the rectangle to the baseline and make it perpendicular. What you now have is a giant "ruler" that you can use to align streets.

Next step is to align streets along this "ruler". Position the ruler over a street, select the two endpoints of the leg of the ruler and all the points of the street, and press L. All the street's nodes will be aligned to the ruler! Lather, rinse, repeat.

Once you have perpendicular streets, the nice thing is that you can shift streets in the X and Y directions independently of each other. If you want to move a street a bit to the right, say, then your new alignment action will not affect streets going from top to bottom.

To align buildings (or other areas) perfectly to a street and make them perpendicular, select two points on the street and the body of the building/area, and press Q.