1. Desktop Dungeons (Rodain Joubert) [Windows, freeware]
Desktop Dungeons is a puzzle-based roguelike with an emphasis on resource management, where the health of your adventurer is restored by walking into unexplored rooms or corridors. You can choose to engage an enemy immediately upon finding them, or save the tougher encounters for later after you've gained some battle experience with weaker inhabitants of the dungeon first.
That is basically the gist of the game, yet once you've spent about an hour or two with it you'd realize that Desktop Dungeons has plenty of hidden complexity to offer anyone who is willing to learn the advanced strategies, bonuses and random elements that Nandrew has implemented in this gem of a time sink.

2. Space Funeral (thecatamites) [Windows, freeware]
Space Funeral tells the story is about a man named Phillip, who is on a journey to reach a place called the City of Forms. You get to recruit only one travelling companion during the entire adventure, meaning that there is very little party management and individual character stats to worry about here. The exercise of grinding for experience is also kept to a minimum, and you can beat the final boss even before either of your party members reach level twenty or above.
The game takes about two hours to play from start to end, but you have to remind yourself to save often because there are no inns or checkpoints to record your progress for you. Should the random encounters becomes tedious over time, players can also use the auto option to have battles resolved for them automatically.

3. The Spirit Engine 2 (Mark Pay) [Windows, freeware]
The Spirit Engine 2 is Mark Pay's self-developed sequel to the original TSE released way back in 2003. Set in a new world, with new characters, new graphics and a rewritten game engine, TSE-2 is a major improvement over its predecessor in every possible way. This side-scrolling RPG doesn't cost you a single cent to download, but will still provide you with hours upon hours of gameplay enjoyment.

4. Rogue Survivor (Roguedjack) [Windows, freeware]
Rogue Survivor is a survival roguelike sandbox game with plenty of depth to explore. You are put in the role of an average person in the middle of a zombie outbreak, and must stay alive for as long as possible.
Each day you survive, you'll level up and become stronger - but tougher enemies will also appear. Apart from finding weapons and barricading yourself into a safe area, you'll need to make sure you have a good supply of food and perhaps some friends to help out. Apart from playing a survivor, you can also play as a zombie and see how many humans you can eat.