Jennifer Laycock, Search Engine Guide, wrote an excellent article on successful low-cost viral marketing with examples. She talks about the customer acquisition cost (CAC) in developing these promotions.
In October I made a post that turned out to be very popular on creating a collection of expert interviews. Another way to approach this is with customer interviews. Customer profiles or interviews are a common marketing practice.
Jennifer Laycock, of Search Engine Guide, wrote about the importance of relationship development and link building. She offers a great example email and points out what the person did right. Successful link development is all about good marketing. Found via Chris Sherman at SearchEngineLand Right Way to Ask For a Link
Good discussion at WebMasterWorld about article submissions losing their effectiveness over the years. Members give some great advice on what works today.