Google has a gigantic lead in contextual advertising with the success of AdSense. Yahoo has the Yahoo Publisher Network which has been in beta for about 2 years and isn't doing nearly as well as AdSense. Microsoft is still trying to get a contextual advertising program rolling.
Ask.com is going to see what they can do by operating a little different than Google.
Ask.com is owned by IAC. They will introduce the Ask contextual advertising on other IAC sites such as TicketMaster.com, Match.com and Evite. This is planned to happen by the end of May. Ads will also be on some of the sites that syndicate Ask.com search results and search ads.
Mid-sized publishers may sign up for the program now. There is suppose to be a self-service platform for smaller publishers later this year.
Ask.com will be providing publishers with tools to balance between ad relevance and page yield and they promise more features to let publishers manage the monetization of their sites.
Advertisers will have separate bids for contextual ads and be able to block sites.
Ask.com is trying to take advantage of some of the things which some don't like about AdSense.
There are more details. Here are links to some articles with more details:
Ask.com to Launch Contextual Ad Network
IAC Advertising Solutions Announces Contextual Advertising Product: Financial News - Yahoo! Finance
Ask.com to launch contextual ad platform - Search Marketing - BizReport