Have a company newsletter available via e-mail. Announce your site via newsgroups. Newsgroups have grown in popularity to over 30,000 in number, and are read by several million people on a daily basis, so choose carefully. Only post an announcement to sites that are relevant to your business, where people will appreciate hearing about your product or service. Join Email lists and use them to annouce your site and business happenings.
Email lists are similar to newsgroups, except that the user gets the messages by email.
This is not spam, because the user has specifically requested to receive the information in question. Utilize this channel, don't abuse it. All billing statements. All outgoing purchase orders. All print advertising.