If you’ve closed all windows in Safari, Chrome, or Firefox (but not quit the app itself), you can quickly open a new window without loading your homepage by switching to the browser and hitting Command-L, the default shortcut for placing keyboard focus in the address bar. The new window that opens will default to an about:blank page, waiting for you to get typing.

This works at least in Safari 5.1.1, Chrome 14.0.835.202.1912843.1834873471.676752.3, and Firefox 6.0.