This is a must have for every flex developer. These two tools will help you alot. Many thanks to all authors/companies of these tools to provide us with some great flex resources!

Flex Style Explorer 2 Style the flex UI components on the fly and export css data.
Flex 3 Component Explorer See all UI components plus code examples in mxml langauge.
Flex 2 Filter Explorer Explore all filter effects and settings. Great tool found on merhl.com.
Flex 2 Primitive Explorer all primitives in flex!