Xfce
Xfce Developer Center
C API documentation for all Xfce core libraries.
developer.xfce.org
Extras Developer Center
C API documentation for Parole Media Player plugins and Exo. Updated daily.
docs.bluesabre.org
Python
GObject API Reference
Python API documentation for developing GTK applications.
lazka.github.io/pgi-docs
Vala
API Reference
Vala API documentation for developing GTK and elementary applications.
valadoc.org