I clicked on the end of two parallel lines and AutoCAD gave me a half circle connecting them!
That's right folks, you can issue the fillet command with any fillet radius set and create a connecting, half circle between two parallel lines by simply clicking on the ends!
If one of the lines is a longer than the other, the endpoint of the  first line picked will be the final length for both lines picked.

Try it and you'll be surprised how many uses for this geometry there are!