The GNOME Development Tools
Anjuta: Intergrated Development Environment
Glade3: UI designer
Devhelp: API browser
Accerciser: Accessibility tool
Nemiver: Graphical debugger
Please DeveloperTools/Installation for installation instructions for various distributions.
There are short and self-contained examples for getting started with GNOME development for various programming langauges on library.gnome.org.
There is a the GNOME Devtools mailing list that can be used for discussion.