Pre-PC History

http://www.multicians.org/

http://www.piercefuller.com/collect/

http://www.sysprog.net/history.html

ENIAC

John W. Mauchly and the Development of the ENIAC Computer First General Purpose Mainframe Computer (All Encompasing 360 circle)
 IBM/360 Circle Picture
 Using the IBM/360 at Newcastle, UK
 Basic Info (from Data General)
Lawrence Wilkinson's Saga of Resurrecting an old 360  
Model 91 console

UNIX

http://www.bell-labs.com/history/unix/

http://www.ehlis.com/adam/solaris/history.html

http://www.oreilly.com/catalog/opensources/book/kirkmck.html

http://www.computerhope.com/history/unix.htm

http://www.tuhs.org/unixhist.html

Early-PC/Altair

http://exo.com/~wts/wts10005.HTM

Apple History

http://www.apple-history.com/history.html

http://apple2history.org/

Windows/DOS History

http://members.fortunecity.com/pcmuseum/windows.htm

http://www.microsoft.com/windows/WinHistoryIntro.mspx

http://www.houlden.f9.co.uk/msdos/dosversions.htm

http://www.houlden.f9.co.uk/msdos/dosversions.htm

http://www.acad.humberc.on.ca/~frig8279/osessay/dos/history

Linux

http://www.linux10.org/history/

http://ragib.hypermart.net/linux/

Others

http://www.computerhistory.org/

http://www.sinclair.edu/classenhancements/cis225e-dmk/histlect.htm

Jim's Computer Garage Museum

UC Davis Computer Museum