I've briefly discussed the Web group Anonymous before, but for more insight regarding the power of the Internet, read Gawker's article about /b/'s campaign to take down "singer" Dahvie Vanity.
Gawker provides some background on 4chan, /b/, and the current campaign slash scandal in addition to some great insight for those uninformed on how this group operates.
The Art of Trolling: Inside a 4chan Smear Campaign, Gawker
How the Internet Beat Up an 11-Year-Old Girl, Gawker
Culture Lesson: Trolling
Technology Lesson: Google Bomb