Gnome is very popular, rivalled by KDE, and is used as the Default Desktop Enviroment on many LInux Distro's.
Gnome is written in C mainly as this is the language GTK+ is written in.
The Gnome desktop enviroment usually comes with the following software:
X window manager--->Metacity
X display manager---->GDM
File manager------------>Nautilus
Widget toolkit --------->GTK+
Terminal emulator ->GNOME Terminal
Text editor-------------->gedit
Video player----------->Totem
Audio player----------->Rhythmbox
CD burners------------->Brasero
Image viewer---------->Eye of GNOME
Office suite-------------->GNOME Office
Web browser----------->Epiphany
E-mail client------------->Evolution
Instant messenger---->Empathy
Archive manager------>File Roller
PDF viewer--------------->Evince
IDE---------------------------->Anjuta
Widget engine----------->gDesklets
Licenses used------------> GPL, LGPL
Developers of OS's often modify the software bundled with the desktop enviroment in the repositories and often the desktop enviroment itself to include branding, for instance Ubuntu.
Here is a screenshot of gnome 2.3 running.
(Click to see full size image)