The new CSS3 property border-image is a little tricky, but it can allow you to create flexible boxes with custom borders (or drop shadows, if that's your thing) with a single div and a single image. In this article I explain how the border-image shorthand property works in today's browsers.
A new layout system to replace floats or display:table or html tables. Based on the W3C's proposed 'CSS Template Layout Module' for CSS3; but using jquery to take advantage of this technique today.
There are web-based generators for almost everything; stripes, buttons, forms, loader icons, etc.
CSS Arrow Please focuses on another niche; yes, totally CSS-based (no images) arrows.
This CSS trick is great for using in tooltips, the generator offers multiple options for customization (the position, color and border values) and auto-creates the related CSS code.