The dark side of SEO, when done correctly, can yield staggering results...in addition to a high risk of getting your domain banned from all the search engines. Some of these techniques include comment spamming and cloaking.
Some link popularity techniques are aimed at improving your business. Others are aimed at destroying a competitors brand or search engine rankings.
Redirect Hijacking Using a 302 redirect from a higher PageRank site you can wipe out a competitor with a lower PageRank.
Sending Bad Karma Some webmasters meta refresh the popularity of banned domains at their competitors domains.
Blogspam Some people use cheap Indian labor to spam comments on blogs, like "nice blog - phentermine viagra texas holdem poker 123". Others use automated bots and open proxies to blogspam. Sometimes people blogspam for a competitor to try to ruin the competitors brand.
Too Much Similar Anchor Text If your anchor text profile is not well mixed your site might be removed from the search index for popular queries. Some people have been known to use many sitewide same anchor text links to point at a competing site to prevent their competitors from ranking in Google for their most important terms.
Community Link NetworksMany community programs work to trade on site size, authority, PageRank, or money, as a currency to gain links from other sites which may not be on topic. Some of these networks can build thousands of links virtually overnight. This can lead to high rankings quickly in some engines, but rapid link bursts which fluctuate may lead to a site being filtered or penalized.
Email SpamLots of high margin sites spam unknowing new webmasters asking for link exchanges. Some people may also spam for a competitor to try to destroy their brand.
3 Way Link TradeSome people set up bunk sites and have you link to their high profit site that does not link out to anything. In exchange they give you a link from a site that may be willing to link to anything to try to build linkage data into the main site. Some three way link trades are legit, but many are not.
Link Farms, Forum Spam, Guestbooks & DirectoriesSome people use cheap labor or bots to spam forums and guestbooks. Many directories have no quality standards and will link to anything. Many greedy directory owners have started numerous directories in an attempt to cash in on the directory craze. Link farms are often similar to directories that do not have quality standards although many link farms may be a bit more incestuous with more cross linking.
Random LinksLinks are nothing more than a citation. If you can think of things you can say, think, or do to make people want to link at your site you can build links. There also might be ways to automate portions of the process using software or social engineering. Well what about quality links
One of the quietest forms of so-called "dark" SEO is rumor mongering. By creating an appearance of authenticity for a particular tactic or method, an SEO can influence lesser SEOs to pursue that tactic. This is especially effective on new or part-time SEOs. In a competitive world, such diversions can secure exlusive access to opportunities, and can be very lucrative.
Take for example public "SEO Guidelines" including such things as ebooks, webpages, open forums, and wikis. By aggregating a community of self-proclaimed SEOs to contribute articles and "expert content", one creates an appearance of authority. Contributors are anxious to add content, because it provides them with some external validation as "knowledgable SEOs", visible to the uninformed public. Consequently that consuming public, including wanna-be SEO webmasters, are deceived into believing that the tactics and methods so publically documented are accurate and complete.
In fact the real competitive tactics are absent from such publications, due to the simple fact that public disclosure would promptly render them ineffective. The consuming public is blind to the inherent sampling bias of the wiki (no real SEO secrets, no real competitive advantages). In some extreme cases, SEOs may publish such public SEO documents for the "public good" only until they are content-rich and popular among the naive, at which point the owner can slap ads all over it and monetized that free content for personal gain. Professional SEOs are wise enough to always inspect disclaimers (http://www.organicseo.org/Organic_SEO_Wiki:General_disclaimer.html) and copyright statements (http://www.organicseo.org/Organic_SEO_Wiki:Copyrights.html) before contributing, to be sure they are professional, fair, and equitable.
Beginners have a tendency to try to get their link on as many pages as possible, without putting much thought into it. A lot of this can be classified as spam. This consists of joining a forum or blog, and posting a message about your new web site and it's url. While your intentions may be good, this will undoubtedly upset everyone on the forum or blog, and force the maintainers to simply delete your message, before it ever becomes an effective link picked up by the search engines.
The proper way to obtain links is to actively seek them at sites where link submittal is encouraged, such as directories.