Pathologic is a puzzle game which does what it says on the tin – it's about paths and it's logical. Though its logic may be the kind that makes you alternatively shout at the screen when you get stuck and whoop at it when you pass a particularly nasty level.
Every level is a maze made up of square tiles, and you can only step on the transparent ones. You move a white circle with the [arrow] keys, and your job is to collect all the transparent circles without stepping on the same tile twice. There are fifty mazes, ranging from "beginner" to "very difficult", based on the number of tiles, though this might not reflect your notion of difficulty, as some of the bigger levels could actually prove easier to solve. One very nice feature is that all but the ten last puzzles are unlocked from the start, so you can freely browse through them and solve them in any order you like.
You'll especially enjoy Pathologic if you're fond of minimalism. The levels are stripped down to a bare stylish minimum, with pretty neon lights and neat geometric shapes. This is definitely a good choice for a game which requires concentration because its clean design offers no distractions. So take a deep breath, focus and remember: whenever a tough level gives you grief, you can just laugh in its face and skip it.