The first purchase I recommend for you is a copy of Syl Arena's excellent how-to book, "Speedliter's Handbook".
http://www.amazon.com/...05X/ref=sr_1_1?ie=UTF8&qid=1325480043&sr=8-1
You should also study Lighting 101 and Lighting 102 on David Hobby's blog.
http://www.strobist.blogspot.com/
Since the built-in flash of the 60D is a commander you don't need to buy a commander flash like the Canon 580EX II or the Nissin Di866 unless you need a very powerful flash. You can save money buy buying a less powerful slave flash. Here are three such flash units, the Canon 430EX II, the Nissin 630 II, and the Yongnuo 656EX.
http://speedlights.net/2011/03/30/canon-430ex-ii/
http://speedlights.net/2010/12/22/nissin-di622-ii-review/
http://speedlights.net/2011/08/28/yongnuo-yn-565-ex-flash-review/
The Canon is slightly more rugged than the Nissin but both are of professional photographer quality. The Yongnuo is less rugged and of amateur photographer quality. The Canon is highest in price, the Yongnuo is lowest.
If you want to do a lot of flash photography both indoors and out I recommend either the Canon or Nissin. If you only want to use the flash occasionally for family photos then I recommend you start with the Yongnuo flash.
Don't buy a Gary Fong Lightsphere - you can get the same effect by using a Tupperware container or the bottom of a translucent plastic bottle shoved over the head of the flash. That effect is to turn the directed output of the flash into a non-directed one similar to what you get from a bare bulb flash. It is a light bomb sending light out in all directions. The softening of light comes from the light bouncing off the ceiling and any near by walls.
The problem with these Tupperware flash diffusers is that the bounce is uncontrolled so you have no control over subject lighting
AND if the ceiling or walls are brightly colored the bounce light is tinted, resulting in a tinted subject. It is virtually impossible to completely correct for this type of subject tinting.
Tupperware diffusers are especially detrimental in a large room or outdoors. Most of the light from the flash is lost since there is nothing near by to bounce off. The result is that the flash will fire at full power and run the batteries down much too quickly.
If you want to get soft light from an on-camera hot-shoe flash then learn how to bounce the light off the ceiling or a near by wall. By doing this yourself you can control the way the light strikes the subject, allowing you to have soft shadows and highlights that you don't get from a Tupperware diffuser. Those soft shadows and highlights are what give your subject a 3D look so you want them.
