There are alot of sites that are not completely "cool" or "mature" but that doesn't matter. All that amtters is the styles - here's some of them.
1.Happypuppy-derives from KBToys websites. Happy colors, cheerful fonts and bright pictures.
2.Quake-o-derives from the game Quake. Dark, moody images containing elements of the middle ages, scary, ripped fonts. Drak pictures.
3.Diplomatic-derives from Macromedia's own website, the style is simple, less elements, more content. Everything is packed inside menus so that the page is not cluttered.
4.Techno-derives from the style of iMacs. iMacs have a lining on the top with a number of "ridges" The line-covering effect was produced then. Includes outlines of squares and other shapes, also has some abstract shapes, has very streamlined images and totally packed in.
5.TechnoShock-derives from a little number of sites. Fast-paced animation with very little amount of pictures, though still supporting the Techno style.
6.Abstract-derives from the famous Factory512. Considered a little text and a lot of images to express the feelings.