Blogging: Most blogging software is free. It doesn't take long to get up and running. And it's a great way to interact with your customers and potential customers. It does take some time to build an audience. In the beginning, you should anticipate spending up to an hour a day promoting your blog, commenting on other blogs and writing engaging posts.
Just a quick note: Local companies can benefit from blogging too. I once interviewed a personal injury lawyer who replaced advertising in the phone book with blogging. He now ranks on the top page in Google for "personal injury lawyer" in his home town.
LinkedIn: If you have a B2B company, LinkedIn might be a great way to make connections. You can demonstrate your industry expertise in their Question and Answer section. Not only will you be known as an expert, answering a prospective customer's question can lead to a sale.
Niche Networks: If you were to Google the phrase "social network and (your industry)" chances are you will find a group of people already talking about your industry and similar products and services. Join those groups and the conversation and start networking.
Facebook and Twitter: I would be negligent if I didn't include the popular social networking sites Facebook and Twitter. There is a good chance that your customers hang out on at least one of those platforms. First, spend some time listening to what they have to say, and then join the conversation.
Joint Venture: Are there businesses in your area that have complementary products or services to your own? For instance, if you have a landscaping business, you might be able to partner with a fencing contractor. Fencing contractors often require that trees be cut down or bushes be removed in order for them to build a fence.
Networking: For all the talk about social media and social networking, offline networking is still extremely effective. Chances are, there are several free or low cost networking events near you each month.