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• Providers
• Google (Provider for Google/Gmail Contacts and Buzz followers)
• Plaxo (Publisher/Consumer)
• Portable Contacts at Plaxo
• Portable Contacts support is part of OpenSocial REST Protocol
• Supported in Yahoo's Application Platform
• Supported in MySpace's OpenSocial APIs.
• Supported by Orkut
• Supported by Hi5 
• Supported in shindig (OpenSocial reference implementation in Java and PHP)
• Supported in partuza (sample social network using shindig-php)
• Microsoft supports Portable Contacts as an output-format of the user's hotmail / messenger address book in Messenger Connect
• Mozilla's Contacts in the Browser extension uses Portable Contacts as its internal data structure for all imported contacts
• The proposed W3C Contacts API uses Portable Contacts as its contact schema 
• DandyID (docs) 
• The OStatus protocol (used by StatusNet and others for federated micro-blogging) uses Portable Contacts to represent profile info 
• Yahoo does not currently support Portable Contacts for its address book, but does have its own OAuth-protected Contacts API.
• AOL does not currently support Portable Contacts (but does have other APIs).
• Consumers
• Synqit (Consumer) Imports contacts into Skype for Windows.
• Cliqset
• Goodreads (for Plaxo import)
• Libraries and Tools
• jpoco: Java library for consuming Portable Contacts (also works on Android)
• Portable Contacts library for django (python) by David Larlet 
• Portable Contacts ruby gem (consumer library) by Pelle Braendgaard  
• Google has released OpenSocial client libraries in Java, PHP, python, and ruby (which include Portable Contacts consumer support)
• .NET Portable Contacts library by Adam Bird 
• Wordpress plug-in to advertise your Portable Contacts provider in your blog's discovery document (by Matthias Pfefferle)
• Plaxo has a hosted API to convert Portable Contacts to/from vCard 
• Magdex has a collection of microformats to Portable Contacts converters
• Chris Messina has a spreadsheet comparing contact schemas of many popular formats and providers