This applet shows a number line with all the simplified fractions between 0/1 and 1/1 with denominator less than or equal to maxdenom. It can be used for many purposes -- open ended number theory pattern activity, check on work with fractions, etc.
The same applet with a split number line is at http://www.geogebratube.org/student/m12722. It works on a smaller screen and also shows the symmetry of the number line before and after 1/2.
An applet with nice visual patterns of the Farey Fractions with denominators up to 75 is at http://www.geogebratube.org/material/show/id/12780.