Every day ProgrammableWeb has a Mashup of the Day featured on the home page and the Mashup Dashboard. Today's is a unique one in that there's a lot more money at stake than usual: it's the Salesforce for Google AdWords application which combines Salesforce.com and Google AdWords.
Hoover's, the business information company, has released their own Hoover's API, aka "HAPI". This new API enables developers to create applications that utilize Hoover's database of 27 million companies and 34 million business executives (details at our Hoover's API profile). This is certainly useful data for a variety of enterprise applications.