Links from Search Engines and Directories

It goes without saying that a link from Yahoo and Open Directory are desirable for commercial sites (despite recent reports about ODP slowness).

Depending on your industry, there are also many vertical search engines and directories that can provide valuable links.

See Danny Sullivan's list of Specialty Search Engines to find such verticals as business.com (financial and business search engine), FindLaw (legal search engine), or WebMD (medical search engine).