Bugzilla is a great resource both for developers and users of Mozilla products. It let's people submit and track bug reports. Microsoft just launched something similar for Internet Explorer 7. At the Microsoft Connect Web site people can now offer feedback on IE 7. The IE feedback project is open to anyone and requires a Microsoft Passport account to login.
Writing in the IE blog, Al Billings said, "The intent of this work is to give everyone a better place to give IE7 feedback and to prepare the ground for future versions of IE later."
Check out the blog article for more details along with some screenshots that show you what it looks like.
Keep in mind that the IE feeback project isn't necessarily for security-related bug reports. While Billings didn't specifically say that security bugs should not be submitted he did point out that security bugs should be sent through Microsoft's preferred channel, which is via email to email@example.com.