If you are using Firefox browser (Firefox 2 – Firefox 3.6+) here is an add-on that helps you to block advertisement in the web mail services of Gmail, Yahoo! Mail and Hotmail.
It blocks ads on the right hand side of the screen when using Yahoo Mail, Hotmail and Gmail thereby expanding your message space.
Moreover, it also helps to load the page faster, so if you have a slow internet connection this add-on will also be useful.
And also it has options for Gmail to hide menu items such as Invites, Chat, Spam count etc. and the ability to highlight messages in your inbox when you mouse over.
You can install the add-on from Mozilla.