Computer programmer Michael Yingling developed a search engine for archived Calvin & Hobbes comic strips. Here are some tips for using it:
Currently the search only looks for EXACT phrases (not case sensitive), so if you're looking for a comic with the words "balloon" and "airplane" you cannot enter them both, or it will search for "balloon airplane" together. Perhaps in the future I will fix this, but it's actually a lot more difficult than leaving it as-is.
There is one exception though! You can search for a DATE and it will find that specific comic, though it MUST be of the format MM/DD/YYYY. So 09/01/1986 will work, but "Sept 1st '86" and "9/1/86" wont - yet.
Link via reddit