Hobby-project wise I spent some time exploring RPG maker VX Ace (goodness what a mouthful of a name) It has a good organisation of the different resources that make up a game; tilesets, sprites, sound, animations etc. The default resources are pretty generic but you could add your own content and use the Ruby scripting engine to create a complex game with it.
Currently I've been studying the data which made up the very first Pokemon games, they hold a special place in my heart due to being the very first gameboy games I owned. It is impressive how much fitted onto half a Megabyte of memory! Many people have developed resources to make Pokemon-like games with RPG Maker so I've been giving them a look.
If I do end up making a monster catching game it will of course have to involve procedural generation :P Pokemon Randomizers are available for practically all the official games; changing the location and stats of all the Pokemon. I don't know of any fangames which create completely random monsters or maps though.
Alas the little script I wrote to make a completely random Pokedex isn't polished enough to post :/ Too much time spent doing productive coding and reading clearly!