Do you remember Clickmazes BoxUp" It's an outstanding series of puzzles from Andrea Gilbert. I never solved all of those levels. But I hope to one day!
A new version by Volodymyr Klymenko is called Rects. It's the exact same concept, but just different levels. There are 36 levels and I've solved them all. However, I was convinced that 33 was impossible. My confession: I used a hint for that.
The first 15 levels are really too easy, but that's okay, because I like feeling smart. Those last 5 levels are superbly designed. Now I have to get back to Andrea's version and not feel as smart while attempting her Nasty & Beastly levels.