Looking at the present list of categories-- Computers/Technology Financial Real Estate Recreation Sports Travel Health Business Arts LegalI get the sense that a few more at least are needed. Obviously, there is some point of diminishing returns, meaning that one doesn't want over-narrow categories, but even so . . . . A site giving relevant links for a given geographic area (one county of one state)--where does that fit? A site on English usage? On induction cooking technology and products? On the museums of a small town? About the planet Mars? An encyclopedia? Given highly flexible criteria, one might shoehorn anything anywhere. But I would suggest perhaps these additions: Reference Science Home/Garden Education Here's another thought: some work to implement, but not, I'd guess, killer difficult: Set up a few rather broad categories, then have a few subcategories in each. All ads would each have to be in some one subcategory (no listings in a top-level category). Show each ad perhaps 50% of the time within its subcategory, perhaps 45% spread over the other subcategories of its category, and perhaps 5% universally. You could play with those values, or--if it's not too big a runtime load--let each user set the percentages for his ad. Or, to avoid the "non-relevant" problem, eliminate the "universal" showing altogether. To make an example--and it's only an example, not a suggestion for the particular divisions used--say there was a top category Education with several subcategories, one of which was Language; a site dealing with English usage could select the Language subcategory and know it would be shown, say, 50% of the time on other sites in that subcategory and 50% of the time across the other subcategories of Education.