IP Address Spoofing

DOS attacks (and DDOS) - the use of Spoofed IP addresses .

• Cockroaches of the Internet - Basically, they are the cockroaches of the Internet, they are intelligent, and they have bigger and more effective weapons than you can ever hope to have.
  A quote posted by bradknowles at 4:51 PM on February 1, 2007 on
  ask.metafilter.com
  This was with respect to spam  and being blacklisted on mail servers.
• WikiP on IP address Spoofing
• Arbor Pravail - stop DDOS attacks
• Spoofed IP Address Distributed Denial of Service Attacks: Defense-in-Depth - SANS Institute InfoSec Reading Room 

The SANS Institute paper, which I cannot re-post a quote from (they request that you don't),  talks about the methods by which an attacker can, with root access on a Unix system, open a Raw Socket and can assemble a header so that it looks like it is a connection from any source that the hacker wishes.