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Adblock Plus (Firefox only) blocks most adverts, speeding browsing, and reducing the visual clutter on many sites - banner adverts, adsense etc..  Some website owners object to its use, anticipating reduced revenues.  There doesn't appear to be much downside from the user's perspective.

If it is widely used, it may prompt a conflict between those trying to display advertising and those trying to block it.  With the default settings it doesn't manage to block ads on Google, for example.  It's another technological arms race waiting to happen, where people trying to catch our attention vie with people helping us filter what we see (email spam and comment spam).

Mike Masnick from Techdirt describes how people have been able to raise their profile by giving their content away (their music or videos), and then sell scarce goods on the back of it (their appearances or premium merchandise).

He envisages large changes in the music and video market as the content becomes more of a trailer for how much fun you can have at a live concert (for example).  Where freely released videos of gigs are the band saying "Look how much fun we had at this gig! Why not pay to come to our next one?"