AdForum *This site allows visitors to sample work for hundreds of advertising firms. Only catch is that you have to be a member (fee) to see the good stuff.
Advertising Age Datacenter *Offers reports and rankings of leading international advertising agencies, top media companies and top magazines. Some areas may require registration (free) to access.
Agency ComPile *Interesting US and Canadian directory that allows for flexible advanced searches by such variables as agency name, by clients and the agencies that handle them, agency type, geographic area and several more. Requires registration (free) and cookies enabled.