In this community post I want to put links to some highly engaging games and interactive environments (freely available) which help people explore high-brow concepts in both programming and mathematics. If you know of any other fun place to visit, simply post below. Have fun! Lean: adam.math.hhu.de/ A collection of highly engaging programming puzzles which help you learn how mathematics is proved and formalized. You will never see in the same way the hardness of 5*7=7*5 anymore. NandGame: nandgame.com You make your own processor from scratch (even NAND is made from simpler circuitry). Highly addictive and super fun! Quantum Flytrap: lab.quantumflytrap.com/ This is where you will finally get some understanding of the quantum computations. Colorful, limitless, engaging and mathematically very deep. Scratch: scratch.mit.edu Learn programming the fun way. Euclidea: euclidea.xyz Learn how to perform constructions with ruler and compass. It used to be part of the school eduction but now it's an app. Fun and very informative. Planarity: jasondavies.com/planarity/ Try to find a graph embedding which shows that it is indeed planar. Golly: golly.sourceforge.io The ultimate cellular automata explorer. If you want to dig deeper: SageMath: sagemath.org It's a powerful computer algebra system with Python syntax. It's ideal for courses and the syntax of SageMath is much closer to regular mathematical discourse. I likea teaching courses with SageMath. GeoGebra: geogebra.org You can do proofs, calculations and interative applets. It's a fun way to learn many different facets of undegraduate maths. Wolfram Demonstration Projects: demonstrations.wolfram.com A collection of mathematical applets which can help you understand very sophisticated topics. You don't need Mathematica to run but you need it to design your own applets. The Mechanics of Proof (by Heather Macbeth): hrmacbeth.github.io/math2001… It's a book with an interactive GitHub repo where you can learn more comprehensively the structure and syntax of Lean. An Illustrated Theory of Numbers (by Martin H. Weissman): illustratedtheoryofnumbers.c… This is a textbook about elementary number theory with applications. And these applications are available from the website as a collection of free Jupyter Notebooks that teach you the key concepts. Graphical Linear Algebra: graphicallinearalgebra.net In illustrated and to some extent interactive way of learning deep concepts in algebra. HomotopyContinuation.jl: juliahomotopycontinuation.or… This is an incredibly beautiful and interactive (with Julia) way of learning some highly complex ideas from algebraic geometry (connected components, homotopy continuation method) and with applications to very special algebraic systems which come from applications. You can learn Julia on the way. Mathamaze: mathamaze.co.uk It's an experience by Helena Verrill. Beautiful geometry, tilings, patterns. Simply wonderful. Hard to explain: n: the way of the ninja: thewayoftheninja.org If you think mathematics is hard. Learn the patience the hard way. Warning: highly addictive HyperRogue: roguetemple.com/z/hyper/ You can learn hyperbolic geometry in a rogue-like environment.