Web communities tend to form because new visitors stumble across the site and like what they see. They may have come from a search engine, a link on another site, a friend's recommendation, or even press coverage.
Rather than letting nature take its course, some community builders include "viral" mechanisms to speed up the process. Perhaps the best example is a "tell a friend" or "e-mail this page" link. In my experience, these don't get many clicks, but the quality of the referral is usually excellent since it's targeted by someone who knows the recipient.
What clever ways have you spotted (or used) to use your members/visitors to bring new visitors to the site?
Allow deep-linking to posts and make sure the board is SE-friendly.
I think most people come to my forums because of a link or because they found it via a search engine. As far as the latter is concerned, keep the forums clean. Posts on my forums come up on the first page in Google for just about all the important keywords.
Of course, you always keep people because of the content they find there.
On thing I've toyed with but haven't tried is a "Link to this Page" button that would generate an HTML link using the page's Title as link text and the appropriate URL. I'd guess it would get even less use than an "e-mail a friend" link, but even one-in-a-million would have substantial linkage impact over time.
<added> [vkaryl... maybe you are right about jargon. "viral marketing" [google.com] is a fairly common term that has been discussed here before, though a "viral community" does sound a bit ominous. ;)] </added>
<added> [vkaryl... maybe you are right about jargon. "viral marketing" is a fairly common term that has been discussed here before, though a "viral community" does sound a bit ominous. ;)] </added>
"Viral marketing" and "guerilla marketing" are the established terms for the answers to pretty much every single "how do I get more people to visit/click/buy?" question posted here. I keep finding that offline marketing techniques can be easily adapted to online purposes.
In response to the original query, it seems like "tell-a-friend" is so effective that people don't look for alternatives.
In my case I have a topic website that's been up for awhile and is well indexed and travelled. I've recently taken to providing links from some of the higher traffic topic pages into the discussion forums for the sole purpose of discussing that topic further.