Portal 1.5, a Portal remake using the Portal 2 engine, textures, and models, also spackling in some minor retcons. Not a terrible waste of time, as it is an interesting experiment. (pun intended)
Escher Technologies, a mod by the same guy, about an Aperture-esque laboratory with levels based on the art of MC Escher. In it’s early stages, so it stll looks like a Portal ripoff so far, but I hope it improves.