You can go to:
http://www.ucomics.com/calvinandhobbes/
This site runs C & H every day.... they are currently running the strips from December 1994, in which the calendar dates/days are the same as December of 2005. So when you check it out on Sunday, you'll get the Sunday C & H from 11 years ago, etc.
They do archive the strips by month as well, I'm not sure how far back the archives go...
Incredible, non-stop-high-quality comics for 10 years straight... In light of the fact that it never had a "down" period (as Peanuts did, in terms of overall quality), C & H probably IS the greatest comic strip of all time (of course, Peanuts ran for 50 years, C & H only 10... I guess we could excuse Schultz for an occasional down period).
I'll be purchasing the Complete C & H very, very soon. Should be required reading for everyone...