Single-image, PNG-based, fluid rounded corner boxes with support for borders, alpha transparency throughout, gradients and patterns. The potential downsides are the image file size, IE 6 degradation and vertical scrolling.
http://www.schillmania.com/content/projects/even-more-rounded-corners/

Based on Nifty Corners By Alessandro Fulciniti, Stu Nicholls demonstrates nifty corners with snazzy borders - without any images! And its cross-browser compatible.
http://www.cssplay.co.uk/boxes/snazzy.html

If the element you want to wrap the rounded corners around is a DIV, then there is no need to change the old HTML code with this technique. Simply change the CSS for the DIV.
http://virtuelvis.com/gallery/css/rounded/

CSS Browser Selector is a clever technique to help you with browser-based CSS hacks.
http://rafael.adm.br/css_browser_selector/

Tutorial and code for in-line external link backgrounds in CSS.
http://green-beast.com/experiments/css_link_backgrounds.php

CSS tutorials from W3 Schools, a web developer's portal, with tutorials and references relating to a number of web development subjects. It was created and is owned by Refsnes Data, a Norwegian software development and consulting company.
http://www.w3schools.com/css/