Writing and posting press releases at free
pr sites can generate some traffic, but only if you have something newsworthy or new. If you don't, make some news by developing an online survey & posting results, holding a contest, celebrating your company anniversary with a giveaway or special event, or announcing a public speaking engagement.
And while public speaking isn't "online" advertising and harder to measure (the stats often show "bookmark" or "no referrer" spikes in traffic), public speaking or presenting usually tends to boost site traffic during the week of the speech/presentation. Local groups often look for luncheon speakers, convention speakers, etc. Do it for a low cost (or even free), with the stipulation that you can have a slide with your URL on it, or have someone read your short bio before you speak with your URL.