Canon 100mm f2.8 USM
I bought Canon’s 100mm f2.8 USM macro lens. It is a fun toy. After a couple hours of wandering around shooting things with it, I finally copied all the shots to the computer so I could look at them. Looking through them was lesson on depth of field. In particular, the exact edges of the in focus areas are not always precisely clear through the viewfinder. This matters a lot more shooting small things than it seems to when shooting landscape shots.
This also is only my second prime lens (the other being the 50mm). I need to shoot more with prime lenses. It can be educational.