Our API directory now includes 54 calendar APIs. The newest is the InfoPark Web Calendar API. The most popular, in terms of mashups, is the Eventful API. We list 47 Eventful mashups. Below you'll find some more stats from the directory, including the entire list of calendar APIs.
In terms of the technical details, REST and XML lead the way. There are 39 calendar REST APIs and 10 calendar SOAP APIs. Our directory lists 39 calendar XML APIs and 25 calendar JSON APIs.
The most common tags within calendar are 14 events calendar APIs, 8 social calendar APIs and 4 reference calendar APIs.
On the mashup side, we list 28 calendar mashups. We named GoTime as mashup of the day in October 2011.
For reference, here is a list of all 54 calendar APIs.
30 Boxes API: Calendar service
Anniversarator API: Annual event calendar service
AOL Calendar API: Online calendar service
Appointment-Plus API: Online reservation software
Arts Holland API: Dutch cultural event data provider
Astrid API: Task tracking system
AtMail API: Webmail Service
BankHolidays API: Swedish banking holiday lookup service
Calgoo API: Online calendaring service
Cheddar API: text-based task management platform
ClickBook API: Online appointment management service
ClicRDV API: Calendar and Appointment Scheduling Software
Daum Calendar API: Calendar services
DayDetails API: On-this-date lookup service
Discordian API: Date converter from Gregorian to POEE calendar
Docket Alarm API: Litigation Tracking Service
DoStuffMedia API: Music festival data lookup service
Eventful API: Local events discovery and demand
Futuremail API: Email Scheduling and Queuing Service
Fuze Box API: Internet and mobile communication services
Google Calendar API: Calendar service
Holidays Service API: Holiday dates lookup service
Idsecat Advance Release Calendar API: Idescat publications calendar
InfoPark Web Calendar API: Event calendar service
Jotlet API: Online calendar service
Kickserv API: Online CRM service
LessAnnoyingCRM API: Customer Relationship Management Service
Lifepod Open API: Online calendar service
LiveConnect API: Microsoft services connection platform
MakePlans API: Online scheduling service
MINDBODY API: Universal Business Management Solutions
Mobyko API: Mobile phone syncing service
My School Holidays API: School holiday search service
Notely API: Academic notes and calendar service
Orange Personal Calendar API: Calendar service
OSU Calendar Web Service API: Oklahoma State University event retrieval service
Plancast API: Social event service
Planning Center API: Ministry Planning Tools
Projjex API: Project management and CRM system
Recollect API: Garbage and recycling collection reminder service
Sapo Holiday API: Portuguese holiday reference service
SCHED* API: Social Scheduling For Events
Setster API: Appointment scheduling widget
SnapAppointments API: Appointment schedule management
Socializr API: Event listing service
St. Gregorios Church Calendar API: Easter date calculator
StesCodes Social Event Manager API: Social network calendar management service
TeliaSonera Calendar API: Calendar creation tool
Triumph API: Educational framework and tools
Upcoming.org API: Collaborative event calendar
Venteria API: Social event calendar
Vodafone Address Book API: Access contacts and calendar items on mobile device
When I Work API: Employee work scheduling application
WizeHive API: Collaborative workspace service
You might also want to mention the SuperSaaS API in this post: http://www.supersaas.com/tutorials/api