(woot for open source)
It's called DispEd, get ithere.
You set the shape, size, power, and it ouputs a vmf for you to open and copy/paste into your map.
You can use it so your brushes and displacement textures line up perfectly for making arched street corners and sidewalks etc. You can make some real organic looking shapes, which you can later tinker with to give them a real life feel.
A picture without:
...and a picture with: