... and content modification.
It seems like the war for content control (DRM, implicit EULAs, DeCSS, PVRs, ...) is increasingly extending from the music and movie industry to the internet.
Services built on top of other sites generate a good portion of the conflicts: sites with deep-linking policies, lawyers restricting how their RSS can be redistributed or simply, general questions about copyright and caching/proxying/quoting/converting...
But the client (browser) is also a major friction point, because of its ability to modify the content, as illustrated in the debate over Microsoft's SmartTags and Google's AutoLinks.
Advertisement filtering is just a variation on the usual conflict between content producers or users, and many of the classic arguments apply. It's only more sensitive because ads are the revenue source for many websites.
Ad-blocking can takes many forms. Some browsers support it through built-in popup-blocking or user specified CSS. Extensions can also add the functionality: for example, the various browser toolbars, the AdBlock extension for Firefox, or some ad-hiding extensions for MSN Messenger.
Proxies can also be used to filter out ads (one of our CS assignments was such a filtering proxy). Some go even as far as replacing the ads with more ads.
Of course, there is always the low tech filtering approach: developping ad blindness.
Up to now, popups have been the center of attention, with annoyed users trying to block them and advertisers attempting to circumvent the blocking techniques (which certainly doesn't win them any love from users). But as the various other means of blocking ads become more popular, content providers are going to try and regain control.
Google's text ads (AdSense) are certainly less annoying than many (they still use some screen estate) but even they can be filtered out from most sites by tools like AdBlock. How long until Google goes after AdBlock or implements workarounds?
Digging more into the legal aspect of ad filtering could also be an option.
Will content providers start spreading the "unskippable" DVD scene or "forwarding forbidden" TV shows phenomenon on the web in the form of interstitial ad pages?
Will more content be made available on the web in flash, pdf, closed, obfuscated or DRMed formats? Will we see pirated copies of blogs and other websites, distributed thru the darknet?
One interesting factor is the democratization of publishing. Everyone is now a potential content provider. Is that going to change the balance?
Update (2005/04/03): Matt from "Peer Pressure" has a good post on the multiple personality disorder of the blogosphere, conflicted between desire for control as a user and control as an author: on one hand bloggers cheer for RSS and user scripts; and on the other hand many freaked out with AutoLink.