Ok, I admit it. I can never remember the different browser declarations and correct syntax for all of those snazzy CSS effects. At least once a day I find myself using one or more of these great tools.
Great collection of the best supported CSS3 effects and an easy to use GUI.
CSS Border Radius Generator
Super simple way to see how those border radii will look in the browser.
Great way to easily see how nth-child works.
Border image generator
Just getting into using the border-image selector, this tool helped my understanding of it greatly.
Ultimate CSS Gradient Generator
The CSS gradient syntax has to be the most complicated of the lot, but this great tool makes it easy.
Just like your favorite graphics editor but in your browser. And it creates CSS.
Easy way to find the size and position of sprites, super useful time saver.
Should I Prefix
An overview of currently needed prefixes for CSS features: http://shouldiprefix.com