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There are many vendors of PPC advertising. Of vendors offering the greatest reach (and likely the greatest click-through), Google AdWords delivers the most, Yahoo! Search Marketing (YSM) is second, and Microsoft adCenter is third. Smaller programs exist, some boasting a higher ROI than that of the large vendors. Of these, Ask.com, Kanoodle, and MIVA are quite reputable.
Although advertisers may choose smaller vendors to save money, there can be drawbacks: