Minecraft Modern is the latest (and only updated) version of Minecraft which requires a paid account to download, update and play. Most of the new development projects focus on either creating entirely new programs from scratch that interoperate with Minecraft (such as a bot or server) or modding projects that wrap the client or server and provide bug fixes, new features and enhancements to existing features.
Documentation
There are ongoing efforts to keep reverse engineered documentation updated, but it isn't as easy as it sounds. The protocol generally changes with each release, and both the Client and Server classes get rearranged on each release. Below are links to the current documentation segments, which may or may not be up to date.
Minecraft Classic is the original version of Minecraft, available for free to the public. It's still available off of the main website today and is still played by many people, with an active development community. As it's been around since 2010, and because it's so simplistic, there are many, many programs made to work with Classic - there are over 18+ servers alone, written in everything from C++ to PureBasic to Perl. It's highly recommend if you're planning to do a development project at this point to instead look into the Minecraft Modern specifications.
Documentation
As there is no longer any work being done on classic, the documentation for it is stable. If you create something that works with it, it probably always will.
Source code snippets provide insight into how specific features work or can be accomplished, and by themselves are generally free to use in your own program.
