Internet Explorer remains a widely-used browser, but testing for compatibility can be a pain if you're on a non-Windows machine or prefer to stay inside another browser. The recently-launched modern.IE site helps test sites for IE compatibility, letting you enter any site address and receive suggestions on how to ensure it renders in IE.
The site also includes downloads for plug-ins you can use in Chrome and Firefox, and downloadable virtual machine images to let you test older versions of IE on non-current platforms. Useful resources if you do a lot of page testing. (For more comprehensive cross-platform testing, the site is also promoting a three-month trial of BrowserStack.)