If you haven't already seen this awesome Flex website then check it out.
It has examples (including source code) of all the common Flex 3 components (DataGrid, Tree, Button, Canvas, etc) as well as a TON of other examples (269 at the last count) from various open source projects including Flex Yahoo Maps, advanced Data Visualizations (3D charts, gantt charts, calendar views, etc), Cloud APIs (searching Amazon, eBay, Flickr, etc) and much more.
There is also the original Adobe Flex 3 Component Explorer which contains examples of just the core Flex 3 components.
For examples on styling components check out the Flex 3 Style Explorer site. I also find this list of all the styles for each Flex component useful: Flex 3.0 CSS Properties List.