Everything on dlang.org is fine. The phobos/ and library/ sections are two versions of the same content, generated from the same source files at the same time. The library/ stuff is the future, but not completely polished yet.

The obsolete stuff is dsource.org. Most of it does not compile anymore and has no maintainers. If you are looking for libraries to use, go to https://code.dlang.org/.

Phobos is the standard library. Tango is a D1 projects. Most of it has been ported and integrated into Phobos now. Some people still like the Tango XML library: https://github.com/SiegeLord/Tango-D2