We have been successful in posting whitepapers on a few sites that actually let you do it without a fee.

Also, I found a few places related to computer software that allow you to submit your site to be considered for a site of the week. This one brought in a few hundred visitors to the site.

My company is a software development company. We recently released some code to the Open Source Community and complimented the code with press releases to sites and mags geared toward developers. This was extremely successful in generating traffic. We didn't get any sales, but hopefully when their IT dept is looking for a solution they'll remember our name and suggest our software.

Because we are a small business, I have also found that it is crucial to have as many people as possible participate in forums and message boards, not just the marketing department. Everyone plays a part in marketing.