I'm sure you've had many answers already, but I'll take this
opportunity to plug what I think is the best of the hundreds of
places one can get the constitution online (the best free one
anyway; WestLaw's annotated is even better if you can afford it).
<http://www.access.gpo.gov/congress/senate/constitution/toc.html>
is the Government Printing Office's version with history and
analysis of decisions by the Senate, as authorized by 2 U.S.C. 168,
as well as plain text verbatim versions with all the original
spellings, etc.
For your convenience, I've placed the plaintext version from there
on my site at <ftp://ftp.piclab.com/pub/usconst.txt> and
<ftp://ftp.piclab.com/pub/amends.txt>. (Your amendments may be
out of date if they are older than 1992, when the 27th passed).
-- Lee Daniel Crocker <lee@piclab.com> <http://www.piclab.com/lcrocker.html> "All inventions or works of authorship original to me, herein and past, are placed irrevocably in the public domain, and may be used or modified for any purpose, without permission, attribution, or notification."--LDC