Invented by: ATi
1st use: Radeon X800 (2004)
Mainstream use: 2006
Used for: increasing texture quality of normal maps
Replaced by: nothing yet

3Dc was announced in year 2004 with release of ATi Radeon X800 card. Its open format based on enhanced DXT5 algorithm from DXTC. Compression ratio is 4:1 or 2:1 and its been developed for using with normal maps. Lower models from Radeon Xxxx line have not support (X300, X600), Radeon X700 might have support. All later cards since X1xxx line supports it (X1300 and higher). In OpenGL its done through extension GL_ATI_texture_compression_3dc. Nvidia have support since its DX10 line of cards (Geforce 8400GS and higher). OpenGL support on Nvidia is done through extensions GL_EXT_texture_compression_latc and GL_EXT_texture_compression_rgtc.

More info at:
Wikipedia
Anandtech
Tom'sHardware
Nvidia
OpenGL
OpenGL
Ozone3D
AMD compress tool