maybe you alreay know it , but its good to mentioned it :

http://www.the-underdogs.org/

this site contains tons of abandonware/freeware games and reviews from all times.
it all categorized nicely .
by downloading theses old game which many of them were geniusly crafted : great puzzles , super plots , inresting and unique ideas .

you can learn a lot , and take tons of great ideas for games or for puzzles .