No, I don't do anything but word of mouth marketing... it's worked thus far. I don't want to spend money on advertising a site that will grow just fine without advertising. If the site has good enough content and tools, it will grow reguardless. The traffic that google sends me now (just google) paying at the lowest rate ($0.05 per click) would cost me almost 4000/month to advertise. Google is already sending me almost $4000/month of traffic for free, why would I pay for more.. in a few more months when the site has even more content Google may be sending 2X that.
If my site sold a product or service though, I would pay for advertising... because I would be able to offset the cost per user that I paid for. My site right now only makes money from shear volume and advertising. It's not worth it in my line of service to pay for ads.