Also many commercial sites offer a banner exchange. You may have to post two or three banners on your site for each post of your banners in the exchange. But if it brings more visitors to your site, it might be worth it. Try joining newsgroups. If you are going to use this method to help promote your site though, be considerate.
Many people read the posts for information and get annoyed at blatant advertising.
You can, however, add a tasteful signature file to your e-mail. In this file you can add a few words about your site and the URL in case the reader might want to visit. If you enjoy writing, you can publish your own newsletter or write articles for niche publications. Don't forget to send out press releases every time your company does something newsworthy. Publicity is important.