Grey-Hat SEO Tactics

The following tactics fall in the grey area between legitimate tactics and search engine spam. They include tactics such as cloaking, paid links, duplicate content and a number of others. Unless you are on the correct side of this equation these tactics are not recommended.
Remember: even if the search engines cannot detect these tactics when they are used as spam, your competitors will undoubtedly be on the lookout and report your site to the engines in order to eliminate you from the competition.
It is definitely worth noting that, while it may be tempting to enlist grey-hat and black-hat SEO tactics in order to rank well, doing so stands a very good chance of getting your website penalized. There are legitimate methods for ranking a website well on the search engines. It is highly recommended that webmasters and SEO's put in the extra time and effort to properly rank a website well, insuring that the site will not be penalized down the road or even banned from the search engines entirely.
Grey-Hat SEO Tactics:
Cloaking
There are times when cloaking is considered a legitimate tactic by users and search engines alike. Basically, if there is a logical reason why you should be allowed to present different information to the search engines than the visitor (if you have content behind a "members only" area for example) you are relatively safe. Even so, this tactic is very risky and it is recommended that you contact each search engine, present your reasoning, and allow them the opportunity to approve it's use.
Arguably, another example of a site legitimately using cloaking, is when the site is mainly image-based such as an art site. In this event, provided that the text used to represent the page accurately defines the page and image(s) on it, this could be considered a legitimate use of cloaking. As cloaking has often been abused, if other methods such as adding visible text to the page is possible it is recommended. If there are no other alternatives it is recommended that you contact the search engine prior to adding this tactic and explain your argument.
There is more information on cloaking on our black-hat SEO tactics page.
Paid Links
The practice of purchasing link on websites solely for the increase in link-popularity that it can mean has grown steadily over the last year-or-so with link auction sites such as LinkAdage making this practice easier. (You can read more about LinkAdage on our SEO resources page.
When links are purchased as pure advertising the practice is considered legitimate, while the practice of purchasing links only for the increase in link-popularity is considered an abuse and efforts will be made to either discount the links or penalize the site (usually the sellers though not always).
As a general rule, if you are purchasing links you should do so for the traffic that they will yield and consider any increase in link-popularity to be an "added bonus".
You can read more about purchasing links and where to do so on our SEO resources page.
Duplicate Content
Due primarily to the increase in popularity of affiliate programs, duplicate content on the web has become an increasingly significant problem for both search engines and search engine users alike with the same or similar sitesdominating the top positions in the search engine results pages.
To address this problem many search engines have added filters that seek out pages with the same or very similar content and eliminate the duplicate. Even at times when the duplicate content is not detected by the search engines it is often reported by competitors and the site's rankings penalized.
There are times when duplicate content is considered legitimate by both search engines and visitors and that is on resource sites. A site that consists primarily as an index of articles on a specific subject-matter will not be penalized by posting articles that occur elsewhere on the net, though the weight it may be given as additional content will likely not be as high as a page of unique content.
If you find competitors using these tactics it is not unethical to report them to the search engines. You are helping yourself, the search engines, and the visitors by insuring that only legitimate companies, providing real information and content, appear at the top of the search engines.