Here's a classic: MOTAS

The original game that inspired other point-n-click games like the Crimson Room, if you've heard of it. MOTAS is good because there're 19 levels (so far) and it consists of all sorts of frustrating puzzles and mysteries you need to solve in order to "leave" each level. Plus, the cookies'll remember which level you were playing in so you can leave and come back whenever you want.

Disclaimer: highly addicting. I am not responsible