For Internet Explorer users:
Open a text editor.
Paste these two lines in it:
.oM {display: none;}
.u7 {display: none;}
Save as gmail.css
Open Internet Explorer.
Go to Internet Options
Right down click „Accessibility“ button.
In the popup window, right at the bottom tick „Format documents using my style sheet“
Choose the previously created file gmail.css with the „Browse…“ button.
Save with OK button.
Click again OK on the next window.
Restart Internet Explorer.
Enjoy ad-free gMail.
(Tested on IE9)
Users of Google Chrome, Opera and Firefox are intelligent enough to find the right extension or add-on.