None!
All of the major search engine services have evolved to a level of sophistication whereby they no longer tolerate any "tricks". Tricky webmasters were in their heyday in the early days of the Web, but they have lost the war with the modern search engines whose algorithms are constantly changing to ensure the most meaningful results.
As we say elsewhere, nSiteful's "secret" to success in getting excellent placement for our clients' sites is not a secret at all. We develop Web sites with proper HTML code, user-friendly and crawler-friendly navigation systems, and meaningful Web pages. We pay attention to page sizes. We use descriptive page titles for every page in a site. We use structural HTML elements in every page. And we avoid pages that rely too heavily on graphical text elements, because crawlers cannot read graphics.
For a glimpse of how search engines work, you may want to read the
"Guidelines for Webmasters" page at MSN.com (opens in new browser window).