[easy] [medi] [hard] [fuck you] -- I'm not entirely sure about: [netlet] [mathlet]
[-day] [-week] [-month] [-year] [-never] To completion

Starting off:
Hello world
Fizz Buzz
Data Structures (Stacks, Heaps, Binary Trees, etc)

AI:
[medi-week] Neural Network
[hard-months] Tensorflow Clone

Algorithms:
[medi-day] Custom Allocator
[medi-day] Custom Compression Library
[easy-day] ROT 13, ROT 47, ROT with any series
[hard-week] Vectorized SIMD Matrix Multiplier
[medi-week] Printf Suite Implementation
[easy-day] String Manipulation Library
[medi-day] Pathfinding: Dijkstra's Algorithm
[medi-day] Pathfinding: A* Search Algorithm

Applied Math and Problems:
[mathlet] General Lambert's-Problem Solver
[hard-week] Solution to the 8 Queens Problem

Calculators:
[hard-month] Graphing Calculator
[easy-week] Normal Calculator
[easy-week] Reverse Polish Notation Calculator (Look into dc(1))
[medi-week] Spreadsheet (CSV Read/write) (With Excel features)

Compiler (Or Interpreter) Theory:
[hard-month] C Compiler
[hard-3weeks] Holy C Compiler
[hard-week] Scheme/Lisp Compiler
[fuck you-year] Language Virtual Machine
[easy-week] Custom LLVM Language
[medi-month] YACC Implementation

Networking:
[netlet] E2EE PSK Messenger (either GUI or CLI, preferably detached from servers)
[netlet] HTTP Web Server
[medi-months] IRC Client
[medi-months] Matrix Client
[medi-months] BitTorrent Client

OS:
wiki.osdev.org

Games:
/g/gedg/
learnopengl.com
libsdl.org
raylib.com
sfml-dev.org (C++ with a C binding)
godotengine.org (C++ game engine, terrible scripting)

Rendering:
[fuck you-forever] 3D CAD Software
[hard-months] 3D Vertex Editor
[hard-weeks] Bitmap Raytracer
[hard-months] Media Player (Think MPV & VLC)
[medi-weeks] Music Player Daemon Implementation
[medi-week] Image to ASCII (BONUS: Use Unicode or Color)

Simulation:
[mathlet] Hydrodynamics
[mathlet] Aerodynamics
[medi-week] Rigid Body Physics
[easy-week] Voronoi Diagram (HARDMODE: hardware rendering)

Tools (Usable and Applied Algorithms):
[hard-week / doctoral-serveral year long] Terminal Emulator
[hard-week] Text Editor
[medi-day] Syntax Highlighter (preferably for your own editor)
[medi-month] General Compression Library (Think zlib)
[medi-weeks] Multi-Threaded GREP Implementation (BONUS: Highlighting and line numbers)(Hint: use pThread and AIO)
[medi-weeks] Vector/Bitmap Image Manipulation Program