Whenever you are using a tab based application, such as Firefox or IE7, try clicking on a tab using your scroll wheel button. Usually the the action invoked is the tab being closed. Additionally, on browsers especially, the scroll button can be used to open new tabs when clicking on bookmarks or links.

Since scroll mice are a given today, this type of functionality is commonplace in many applications. I have noticed on smaller, text editing applications similar functions used to manipulate tabs, which leads me to believe a lot of other applications support this behavior.

Give it a try in some of your applications, there might be a handy shortcut you never new about.