Alternative Contextual Advertising
If you're thinking about alternatives to Google and Yahoo's contextual advertising then you may want to consider Kontera. According to Revenews Kontera uses Adlinks instead of Ad units. These Adlinks "highlight particular words already in a publisher’s text and present ads in a floating tool tip, only when a user mouses over." This strikes me as an excellent way to minimize clutter on your web pages while maximising relevancy to content.
Joel Comm of Revenews has experimented with Kontera and has found revenues to be encouraging, although there seems to be some ambiguity about whether Adlinks on the same page as Adsense are in breach of Google's TOS.