Here are a few free options:
Social media. You don't have to be everywhere in the social media world (in fact, it's better if you're not). Choose a couple things to really focus on like Twitter and Facebook. Add to the community, post fun things (not just business stuff) so that people can tell your company has a personality, and use it to promote unique things with your company.
Blog. I think just having a blog about what your company does wouldn't be interesting enough to attract people. Like you said, you have a good, interesting topic, so just do posts on that. Those writers who do features and stuff—their articles could be put on the blog. The point is, you provide useful or interesting content and drive people to the site. Then, you can leverage that traffic to make announcements about your company and highlight things.
Article marketing...but you already knew about that one, apparently. Ezine articles and Helium are two free places to submit articles, but there are tons of others, too. If you have a blog, you can guest post on other blogs, especially ones that are targeted at your niche, which could be valuable, too.
Remember, each of these things requires some time to really get going and offer a lot of value, so don't get frustrated when you aren't seeing huge results in the first few months.
Hope that helps at least a little!
I publish a blog called The Fresh Standard, which aims to equip, inspire, inform and support entrepreneurs. Check it out at www.thefreshstandard.com.
These are actually great suggestions that you can probably try. There's a lot of great ways that you can actually market your site online. You can even consider submitting your website to great directory sites aside from article submissions. Try every option you can think of, there' s actually no harm in trying. ;)