General purpose:

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

Particularly for computing terms:

 * [Google Groups][4] for Usenet archives (1981 - present)
 * [DSpace@MIT][5] for the CSAIL archives (1959 - present)
 * IETF's [RFC][6] archive (1969 - present)
 * [PDP-10 software][7] archive (~1967 - ~1990), for old source code
 * [Tech Model Railroad Club dictionary][9], TMRC [1st & 2nd editions][10] (1959, 1960)
 * [The Jargon File][11] and [old][12] versions (1981 - 2003)
 * MIT's [The Tech][13] newspaper archives (1881 - present)
 * [Bitsavers][8]' Software and PDF Document Archive (misc. dates)


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