There are enough distractions on the Web before we even get to the overlays that some sites hit you with, asking you to register, take a survey, sign up for a newsletter, or some-such. And while most overlays put the close button in the upper-right corner, some diabolical Webmasters have become masterful at disguising the X button that lets you close the overlay and enter the site.
If you use Chrome, the BehindTheOverlay extension can help you in your battle against overlay pop-ups. The extension simply installs a button to the right of Chrome's URL bar that you can click to close an overlay that you encounter. BehindTheOverlay just works; it does not require you to sign up for an account or restart Chrome.
And if you head to Tools > Extensions in Chrome and scroll down to the bottom of your list of extensions, you'll see a Keyboard Shortcuts link. Click it and you'll be able to assign a keyboard shortcut to BehindTheOverlay you can use to kill overlays.
If you'd like to test it out, the developer has a far-from-exhaustive list on GitHub where the extension has been tested to work. And on BehindTheOverlay's page in the Chrome Web Store, the developer states that a Firefox version is coming soon.