Mirror Sites and Duplicate Content -
Google has a particular aversion towards spammed web site content and Google penalties for mirror sites reflect this.
Hidden Text and Keyword Stuffing -
A common technique for attempting to inflate search engine ranking is to hide multiple blocks of descriptive text and/or keywords within a web page's HTML source code (Keyword Stuffing).
Link Farms -
Web pages created for the purpose of providing outgoing links to other web pages, usually without regard to content or subject are frowned upon. Linking to a Link Farm can have adverse effects as well.
Employing such techniques will rarely prove to be of any benefit to search engine rankings and are likely to get you banned from a search index all together.