The effigy of Liberty, in various forms.

edit: link fixed

Mercury

@boettiger
Common misconception. The winged cap Liberty is wearing symbolizes freedom of thought. Although Mercury shares the same visual metaphor, the head on the dime is just a variation on the theme of liberty represented on all the previous dimes. The confusion has caused the name “Mercury dime” to come into common usage, but it’s more properly known as the “Winged Liberty dime”.

Check this out…everything you could possibly want to know about the history of the dime, courtesy of wikipedia

http://en.wikipedia.org/wiki/Dime_(United_States_coin)