Pagerank search
Useful to select the pages on which to ask for adding one's link because the results are ordered according to the pagerank of the pages.
http://www.seochat.com/seo-tools/pagerank-search

Find the good keywords
Listing of the pages pointing towards one's competitor pages, and thus likely to point towards our pages.
http://www.goodkeywords.com, http://www.webconfs.com/backlink-builder.php, http://www.linkhounds.com/link-harvester/backlinks.php, http://www.linkhounds.com/hub-finder, http://tools.seobook.com/backlink-analyzer

Co-op Advertising network
Banner exchange system that provides hard links for dynamic websites.
Comment: Google and the other search engines don't like that very much.
http://www.digitalpoint.com/tools/ad-network/

One-way link verify
automatically checks if a link pointing to our page has been kept or removed.
http://www.webuildpages.com/tools/one-way-links.htm

Links purchase and sale
The Pagerank transmitted when buying those links can help rankings
http://www.buyselllinks.com, http://www.linkadage.com, http://www.textlinkbrokers.com, http://pradnetwork.com