Since Tim hunt wrote today (http://moodle.org/mod/forum/discuss.php?d=113802#p500215) that there was no feature request for this in spite of so many wanting this, I'm taking the "freedom" of putting this in, without searching.
There are many people that have wished for this feature. People that want to easily demonstrate how good Moodle is (and quiz is one of the very good things). Teachers that would like to give prospective students a chance to do a diagnostic test before signing up.
(It would be good to have the possibility even for testing/demonstrating other modules like forum, glossary, wiki)
What would it do?
When a guest clicks on a quiz:
it checks if the quiz is allowed for guests
NO- gives the usual message
If self registration is allowed, points out the advantage of registering and offers a link to that.
Says that it is creating a temporary ID: guest
=an unused number
the the person can use for 1 hour/day/week that the person can use while doing the quiz (and to come back during that period)
THEN the quiz engine would work as usual, saving results and so forth, but not influencing the gradebook etc for real IDS.