Nikon Digital SLR Guide, 1999-2010

High-end (Professional) - FX/Full Frame sensor
Nikon D3, August 23, 2007
Nikon D3X, December 1, 2008
Nikon D3S, October 14, 2009

High-end (Prosumer) - FX/Full Frame sensor
Nikon D700, July 1, 2008

High-end (Professional) - DX sensor, high resolution
Nikon D1, June 15, 1999
Nikon D1X, February 5, 2001
Nikon D2X, September 16, 2004
Nikon D2XS, June 1, 2006

High-end (Professional) - DX sensor, high speed
Nikon D1H, February 5, 2001
Nikon D2H, July 22, 2003
Nikon D2HS, February 16, 2005

High-end (Prosumer) - DX sensor
Nikon D100, 21 February 2002
Nikon D200, 1 November 2005
Nikon D300, 23 August 2007
Nikon D300S, 30 July 2009
Midrange - DX sensor
Nikon D7000, 15 September 2010

Midrange (Consumer) - DX sensor
Nikon D70, 28 January 2004
Nikon D70S, 20 April 2005
Nikon D80, 9 August 2006
Nikon D90, 27 August 2008
Nikon D5000, 14 April 2009

Entry-level (Consumer) - DX sensor
Nikon D50, 20 April 2005
Nikon D40, 16 November 2006
Nikon D40X, 6 March 2007
Nikon D60, 29 January 2008
Nikon D3000, 30 July 2009
Nikon D3100, 19 August 2010