Syntax
Using {{reflist|2}} will create a two-column reference list, and {{reflist|3}} will create a three-column list. Three-column lists are inaccessible to users with smaller/laptop monitors and should be avoided.
Using {{reflist|colwidth=30em}} will allow the browser to automatically choose the number of columns based on the width of the web browser. Choose a column width that's appropriate for the average width of the references on the page.
CSS used
.references-small { font-size: 90%; }
Note: Multiple columns currently render properly only in Mozilla Firefox and other Gecko-based browsers and Safari 3, though the feature is included in CSS3 so it should work for a larger number of browsers in the future.