While looking around for data sources for a couple of web projects I kept on stumbling across Yahoo’s API set.  Some of them are fairly popular such as flickr, maps, and music but there are quite a few other useful ones that Yahoo Provides. 

• Answers - Provides access to questions by category, raw question queries, and questions by user. A good use for this would be if you have a product or FAQ page that provides general information about some product or application. 
• APT(Advertising Platform) - Helps to automate tasks of buying and selling advertising, ideal for agencies and publishers. An agency could build a custom ad manager tool for their clients through this API.
• BOSS(Build your Own Search Engine) - open search web services platform. Basically this gives you access to Yahoo’s search index on your own site, pretty nifty.  Techcrunch just implemented with Boss and they seem to like it quite a bit so far. 
• Delicious - Since delicious is now part of Yahoo this is one of their apis as well.  Delicious is very easy to integrate with since their API just uses simple HTTP requests. Could use this for displaying the most recent articles that were saved in delicious related to some topic.
• Finance - Pull stock information for companies and industries. 
• FireEagle - Access to their location based social network.  You could build an AIR application to integrate with FireEagle or integrate a part of a web site with this API.
• Flickr - duh.
• GeoPlanet - Originally launched as the “Internet Location Platform” it’s goal is to geolocate data across the web. 
• HotJobs - job creation and management with Yahoo Hot Jobs, candidate search and view as well.  Can also utilize the RSS feed to display job postings on your own site.
• Local - Provides access to local business information and user reviews as well as traffic alerts. 
• Mail - Use this to build a better interface than Yahoo’s mail interface.
• Maps - Similar to the google maps api.  Provides flash, ajax, and map image access.  Provides more location based information than Google maps such as local listings, traffic, weather, upcoming events, and flickr photos. 
• MediaPlayer - Put a link to an audio file on your site, this will turn that link into an embedded media player on your site. Very cool for dealing with sound clips and podcasts. 
• BluePrint(Yahoo Mobile Widgets) - Write your mobile app once, blueprint ensures it works across hundreds of devices from the basic to the iphone. Not sure how reliable this is but awesome concept.
• Music - Gives you access to catalog data of artists, albums, tracks, videos, and user ratings.  Can even embed a video player on your site if you want.  
• OAuth - Use OAuth through Yahoo for authentication.
• Pipes - Who could forget the short lived Yahoo Pipes?  Pretty cool concept actually.  Basically you can easily create a mashup rss feed.  For instance you could pull stories from digg, slashdot, and techcrunch with the category “mac” and that would be published in the RSS feed which you could then display on your site or put into google reader.
• SearchMonkey - Put Yahoo search on your site but customize the results to be more appealing to your application.  You have control over various meta data pieces and some of the layout of a search result.
• Travel - Lookup airfare, hotels, destinations and much more through the travel trip planner. Kayak.com is also a really good resource for this.
• Traffic - Real time traffic alerts through a feed or over the REST Api.
• Shopping - Search through the millions of products on Yahoo’s shopping site. Also has comparison shopping and user reviews available. 
• Events(from upcoming) - integrates with the popular event site ‘upcoming’ allowing you to get event information, add events, edit events, add tags, search, get groups and much more information. Very useful to pull in related events for your site such as all “merb events” and place in a sidebar. 
• Weather - Access weather data for any location for free.  Provides detailed information such as humidity, wind direction and speed, visibility, barometric pressure, etc. 

I left out a few such as the contacts API, and the Yahoo authentication API.