Just a few years ago, most search engine marketing (SEM) spending went directly to the search engines. Today, SEM is becoming a more mature market like traditional advertising in which agencies play a substantial role. Indeed, while just 31% of search marketers use an agency to manage their SEM campaigns, those marketers account for 51% of the total spending on paid search, according to JupiterResearch. This marks a significant increase in the spending directed by agencies over the past 18 months. Decide Interactive/24/7 Search, Did-It.com, Impaqt, Performics and AvenueA/Razorfish Search are closely grouped for top ratings in paid listings management. iProspect and Impaqt outpace all the other vendors in organic optimization.