This is very important for any company yet is often skipped. Creating a list of people that you can contact at an instant is very powerful businesses. Whats even more powerful is a list of people that WANT to be contacted by you and respect what you have to say. Creating a list isn't enough though; do it wrong and your just another spammer, do it right and your a valuable resource that people respect and enjoy hearing from.
Social media - Facebook, twitter, digg, mixx, myspace, etc...
All these sites encourage relationship building. They are a method of furthering your brand and building a following. This can be very powerful for driving traffic and have the ability to put you on the map. If you create a very interesting / controversial article or video that people are interested in, it can spread like a wild fire through the internet giving you crazy amounts of traffic.
SEO - Search Engine Optimization
Ok, its not really 'marketing' per say, but SEO can help bring customers to your site by moving you higher up in the search engines. This can bring lots of 'free' traffic and can boost your sales month after month.