Sapphire is a server emulator for the retail client. While many things aren't implemented, a great deal of information related to game files, packet information and other misc information can be found throughout the codebase.
XL is a retail launcher replacement that also provides additional functionality to the game such as:
Launching tools like ACT when starting the game
Discord Rich Presence
Integrity check for game files
Chat bridge to Discord (and notifications such as duties, retainer sales, etc.)
Lumina is a .NET (Core) library aimed at providing fast and easy access to game data.