This is admittedly elementary to anyone who has dabbled in Javascript, but it's still an interesting thing to know for the uninitiated. Basically, any passwords stored in web browsers are RIDICULOUSLY easy to view. This trick should work on any web page with a saved password, but Gmail seems like it would be the most relatable, without further ado...

Go to the login page screen for Gmail, where you enter in your user name and password. If the password is stored, you can see what the saved password is by entering the following code into the address bar:

javascript:alert(document.getElementById("Passwd").value);

Enter in the previous statement directly, press enter, and voilà, password shown. You can do this on any web page, so long as you adjust for the actual name of the password input field. For instance, the Gmail login page accepts passwords in the following field:

<input type="password" name="Passwd" id="Passwd" size="18" class="gaia le val"/>

As you can see, the "id" value they used was "Passwd", hence Passwd is what you need to enter into getElementById("Enter ID Here"). Just change that accordingly for every web page you visit, and you should be golden.

Just remember kids, messing with a friend's email account is all fun and games until you notice he subscribes to a Frolicking Furry Fetish Fan club and things get awkward. Drew. Anyhow, don't be malicious!