Much has been made of intrusive ads and ad blockers. To some extent. this about the annoyance of pop-ups (actually not that common in my experience with education sites). More importantly, sites can be adding cookies to your browser and collecting information you might not be willing to share.
Ghostery is an extension you can add to your browser to give you information and some control over the information being collected. I am against blocking ads in most cases because I see the use of ads is a decision to be made by the content creator and not the content consumer. I do think it is important that consumers have some awareness of what cookies are operating on their browsers.
The following two images shows Ghostery in action. The first shows the return from my site which does display a Google ad on most pages and collections Google analytics data on the use of my site. The second images shows the Ghostery return when using the popular education resource site hosted by Richard Byrne. I knew this site was funded by the display of many ads and wanted to see the difference.