Ever find yourself moving the mouse up to click in the address bar of a browser, and then accidentally clicking somewhere IN the current URL, and having to highlight the whole thing and delete it, and it taking an age? (Can you tell yet that I do this *often*!!!)
In Firefox, Safari and Chrome you just press Control L to jump to the address bar, and highlight the whole thing. You can then just begin typing and the new URL will go straight in to the address bar. How cool is that?! Not sure if it works on the Mac/Linux etc, but it works on Windows.
In Internet Explorer (at least in IE 8) you get a pop-up on the screen asking you to enter the URL you want IE to open. So essentially the same thing done slightly differently.