I always keep an eye on who's advertising where. It says a lot about the state of the company and/or it's growth stage. It also says a lot about the network publishing those ads - I regularly hover over ads to see what networks are serving what ads on which domains.
With that, three companies that are, based on my scientific web viewing, advertising very actively:
1. Flock - they actually have a great banner unit. Appearing on tech-related or social sites. 2. Disaboom - a network for the "millions touched by disabilities." They are advertising on any social, news or health properties. 3. Microsoft Live- perhaps the most interesting ads. MS is promoting Live as a dating destination that extends from the net to the mobile phone.