What are marketing's most successful lead sources?

Of course the answer here is "it depends." It depends on who you ask inside our companies and this really underscores the point about aligning with sales. Each sales team has very different goals. For us, we really focus on creating the most pipeline per dollar spent. To achieve that we use a mix of paid search on the major search engines, email, outbound tele, Pay-per-lead and Cost-Per-Inquiry (CPI) programs utlizing both content and webcast promotions with 3rd party media and partners. Each of these programs has pros and cons but in general, the programs that generate the latest-stage buyers are the ones producing the highest ROI. These include Pay-per-Lead programs and "Solution Directories" where prospects are looking for a demo or seeking to be contacted to a sales person at your company. I call these "sales-ready" leads and they both require less follow-up and also convert to sales very quickly. The downside is they may be more expensive, have a limited quantity and require more effort to ensure the leads are a match to our sales processes.