Page cloaking is a BIG one, which basically consists of using server side scripts to determine whether the visitor is a search engine or a human and serving up different pages depending. The script would serve up a keyword rich, totally souped up page to search robots, while giving humans an entirely different page.
Linkfarms are considered black hat. Linkfarms are basically sites which consist of masses of links, for the purpose of getting rankings in search engines and turning a profit (usually off of affiliate program advertisements on the website).
Duplicate content can keep a page from getting indexed, and some people have even reported trouble with entire websites being duplicated by unethical webmasters, causing problems with their rankings.
Spamming keywords, in meta tags, title tags, or in your page's content is a black hat SEO trick that used to work well back in the 90's. It's long since been basically exterminated as a useful trick.
Linking to "bad neighborhoods", or sites that have been banned from search engines for using the above tricks, while not particularly black hat is definitely unhealthy for your own sites rankings.
