General purpose:

 * [Online Etymology Dictionary][1]
 * [Google Books][2], set date range and sort by date
 * [Google Ngram Viewer][3]

Newspapers:

 * USA: [Chronicling America][4] (1836-1922) by the Library of Congress
 * Australia: [Trove][5] (-1954) by the National Library of Australia

Particularly for computing terms:

 * [Google Groups][6] for Usenet archives (also good for slang) (1981 - present)
 * [DSpace@MIT][7] for the CSAIL archives (1959 - present)
 * IETF's [RFC][8] archive (1969 - present)
 * [PDP-10 software][9] archive (~1967 - ~1990), for old source code
 * [Tech Model Railroad Club dictionary][10], TMRC [1st & 2nd editions][11] (1959, 1960)
 * [The Jargon File][12] and its [archives][13] ([also here][14]) (1981 - 2003)
 * MIT's [The Tech][15] newspaper archives (1881 - present)
 * [Bitsavers][16]' Software and PDF Document Archive (misc. dates)


  [1]: http://etymonline.com/
  [2]: http://books.google.com/
  [3]: http://books.google.com/ngrams
  [4]: http://chroniclingamerica.loc.gov/
  [5]: http://trove.nla.gov.au/newspaper
  [6]: http://groups.google.com/
  [7]: http://dspace.mit.edu/
  [8]: http://www.ietf.org/rfc.html
  [9]: http://pdp-10.trailing-edge.com/cgi-bin/searchbyname
  [10]: http://tmrc.mit.edu/dictionary.html
  [11]: http://www.gricer.com/tmrc/tmrc-dictionary-intro.html
  [12]: http://www.catb.org/jargon/
  [13]: http://jargon-file.org/archive/
  [14]: http://www.catb.org/jargon/oldversions/
  [15]: http://tech.mit.edu/browse.html
  [16]: http://www.bitsavers.org/