A method of delivering secondary content for display concurrently with primary content by a web browser running on a user's computer includes running a client-side software application on the user's computer. The client-side software application collects usage data and generates a user profile based on usage data. Multiple items of secondary content are received at the user's computer and characteristics of the items of secondary content are compared with the user profile. At least one of the items is selected based on the comparison and the selected item of secondary content is displayed in a browser window concurrently with the primary content.