[Steam] Binding of Isaac - 80% off ($0.99USD)

Great game, prepare for a lot of 'just one more'!
PCGW
Meta Critic
System Requirements

Key features:
Randomly generated dungeons, items enemies and bosses, you never play the same game twice.
Over 100 unique items that not only give you powers but visually change your character.
50+ enemy types each with the ability to become "special" making them extra deadly but they also drop better loot.
Over 20 bosses.
4 full chapters spanning 8 levels
3+ unlockable classes
Multiple endings
Tons of unlockable items, enemies, bosses and more.