Both of these methods utilize the same functionality: they change the type of text box a password is entered into from a "password" box (masked with asterisks or •) to a "text" box, displaying its contents.
The first method can be done in Safari or Chrome (since they're both WebKit). Right-click or Control-click on the masked password field and select Inspect Element. Within the Inspector, you'll find the input HTML tag is highlighted. One of the attributes there will be type="password". Simply clicking on password will allow you to change it; type in text and, without closing the Inspector window, look at the password field to see your password revealed.
The second method is even easier, and does what is described above, just using Javascript. Drag the text below to your Bookmarks bar or menu. Whenever you're on a page with a password field whose text is hidden, click on it or select it to reveal the password.
javascript:var%20els%20=%20document.getElementsByTagName('input');%20for(var%20x=0;%20x%3Cels.length;%20x++)%7B%20if(els%5Bx%5D.type.toLowerCase()%20==%20'password'%20)%7B%20var%20test%20=%20els%5Bx%5D.type%20=%20'text';%7D%7D
Second, bear in mind the security risk here. Assume you add this bookmarklet to your Safari bookmarks toolbar; anyone who comes to your computer could eventually click on it if they're on a page where your keychain has entered a password. Of course, they'd already have access to the page, if your keychain was unlocked, but them seeing the password could be more serious, if you tend to use the same password on multiple sites.]

