Pretty much from it's inception, the Articles module has provided the ability to filter Articles by Category by including the CategoryID in the URL (e.g. www.mysite.com/default.aspx?CategoryID=30). What I didn't do initially though was make an easy way to select the Category you want to filter by. This changed when I made version 3.1 of the Categories module available. With 3.1, all the categories became clickable links that when clicked on, would append their ID to the current pages URL.

You can see this in action on the All Articles page of this site.