If someone is providing a free service that tons of people use that require some sort of income then ads provide that income (i.e. enough to pay off the server costs). Donations do not cut it and are generally never enough. Very simply ads that aren't intrusive i.e. a top ad and a side ad like on
the site I am a part of (Linked for ad proof) I don't think are wrong and I don't think people should typically be blocking them because they're using our site / services for free and we cannot provide those without any income. Even then, ads aren't really enough.
If you wish to see a lack of free sites or anything providing free content by all means block every ad. In this economy nobody donates.
That said, ads such as popups or intrusive ads that get in the middle of your content isn't so innocent and by all means feel free to block it.