I've just finished reading Bnoopy: The long tail of software. Millions of Markets of Dozens. and I highly recommend it.
The idea of a long tail is not new, but was popularized by Chris Anderson of Wired. The concept contrasts to the traditional 80/20 principle, and essentially refers to the countless situations where 80% of the benefit can not be achieved by targetting 20% of the "problem" or market.
The Bnoopy article uses Excite as the main example. But
Chris Anderson's blog The Long Tail has another interesting article on the long tail at Google (as well as many other interesting aspects of the Long Tail phenomenon).
Another good article is this Tech Central Station article called 'Chasing the Long Tail'
Long Tail Marketing is a blog devoted to the concept of marketing to the tail.
Long tail at Technorati.