Seems that advertising.com is great other big company. Over 600 employes and well spread over the world. Offering advertising solutions for Advertisers and Publishers.
I've used them before and wasn't to happy with the results, ValueClick Media is a much better alternative to AdSense\Adwords if you ask me.
Are you happy with their service? I have never used em for buying ads. Maybe both are big and wide spread [ according to OP ]