We scold Ask.com for not using PPC to promote their brand but we most praise Yahoo! for using Google AdSense for promoting their brand.
A WebmasterWorld thread notes that Yahoo! is using Google's contextual ad program, Google AdSense, to promote its own product. It is important to note, that Yahoo! may not be directly advertising in the Google AdWords program. It may be a Yahoo! affiliate who is doing the leg work here. If that is the case, and it probably is, then I will take away my praise from Yahoo! Nah, I am just kidding. Yahoo! has an affiliate program, that deserves some praise also.
It is also important to note that although I scolded Ask.com for not using PPC to promote their search engine, sem4u posted that he did see Ask.com ads in the Google AdSense program as well. So there you go.
Forum discussion at WebmasterWorld.