Portal Pro is a set of test chambers focusing on solving puzzles locally rather than using complex fling techniques. The goal here is that the solutions are hard to come up with but easy to actually do.
An amazing mod if you ask me. There isn't any story but it does difficult puzzles in the correct way. It's fairly easy to solve a puzzle once you figure out what to do, but figuring out what to do can be really challenging and that makes this mod a lot of fun.