When it comes to SEO many webmasters think of link building in order to boost their rankings, generate traffic and earn money during the process, however the methods used by many people are less than appropriate and the same practices are replicated by many others who want to make a quick buck. When it comes to link building, the end doesn't justify the means.
If you are brainstorming your link building campaign here is a good checklist of things to avoid while optimizing your sites.
Spamming Wikipedia
As many of you know, this particular site has tremendous value for the end user, it has unbiased information which can be used for research because it is factual and not based on emotions and speculations; in fact this is one of the reasons why it ranks so high on the search engines, particularly Google. Many webmasters have thought of editing an article related to their site with the purpose of obtaining a high PR link, the only problem is that in order for your article to get accepted at this online encyclopedia it must be well researched, well written and unbiased; while it may sound easy to do, it is actually quite hard since you have to eliminate sales speech and provide pure information. With all of this said, you might want to reconsider your plans of "spamming Wikipedia"
Incidentally, Wikipedia has been using the "no-follow" attribute for some time now, so your site will not get the link equity you might be after.
Spamming Youtube and other Video Sharing Sites
This is yet another "opportunity" for people desperate for back links. Just like Wikipedia, video sharing sites have implemented the "no-follow attribute" which provides no SEO benefit to your site. Moreover, people hate it when they are having a discussion relevant to the video being watched and all of the sudden someone bluntly suggests a site which is not even relevant to the conversation, if you ever decide to do this you would be killing your SEO campaign since potential visitors now hate your site because you have been spamming it all over video sharing sites.
Spamming Forums
Message board spam has been around for quite some time. Message boards or forums were designed to attract people who would like to share their ideas and opinions with people interested in the same topic, as you see forums are all about content relevancy which is what most "sig-spammers" forget.
Spamming Blogs
If you have ever had the change of starting your own web-log, you may have noticed that several individuals tend to post "comments" which are highly irrelevant to the post and are also filled with a plethora of links. Bloggers dislike this practice very much, that is why blogging platforms such as Wordpress have implemented anti-spam plug-ins such as Akismet in order to minimize this "comment-abuse" Again, building links this way gives your site a bad image.
Spamming Webmasters
It is very annoying to check an email client just to find out that one more webmaster has sent a poorly written and sometimes irrelevant link exchange request, there is a really good chance to acquire strong links this way, however sending machine generated messages to a random list of webmasters will not get you anywhere, in fact, it will hurt you. When building reciprocal links, it is always a god idea to focus on relevancy and quality instead of quantity.
Link Pages (not relevant or useful) - Site-wide reciprocals
When webmasters were caught on the "link-fever" a few years ago, the creation of generic link pages became popular. People added links to a huge list which offered absolutely no value to the surfer, if this is what you are doing then you might want to research new ways to attract inbound links. Exchanging reciprocals with other sites is not a bad idea if it is done the right way, so forget link pages and place those reciprocals on pages which have content and are directly relevant to your trades, this will also avoid site-wide links which look like an effort to artificially create weight and page rank.
Submitting Undeveloped or Poor Sites to Directories (DMOZ, Yahoo Directory, Business.com, Joeant.com, etc)
The sites quoted above provide great SEO benefits if your site gets listed and indexed within their categories, the main reason people get rejected from these directories is because their sites are not fully developed or provide no value to the visitor, if this is the case you might want to add content thinking about the end user rather than the search engines because in the end, the ones clicking your ads and buying your products are not the search engine spiders but the "human visitors"