Presented here for your coding pleasure, a reasonably comprehensive list of open-source web API libraries in various languages. Enjoy ...

Twitter

• TwitterooCore API (C# / .net)
• Twitterizer  (C# / .net)
• jtwitter (Java)
• My Twitter (php)
• Twitter by Felix Oghina (php)
• TwitterLibPHP (php)
• Arc90_Service_Twitter (php)
• PHP Twitter (php)
• Twitter4R (Ruby)
• Twittery (Ruby)
• Twitter by John Nunemaker (Ruby)

Facebook

• Facebook ActionScript API (ActionScript)
• Facebook Developer Toolkit (ASP.Net)
• Facebook Developer Toolkit Addon for ASP.NET MVC (ASP.Net)
• Jay Lagorio's VB.Net Library (.Net)
• FBDK (Facebook Dev Kit) from Microsoft (.Net)
• VS.NET Starter Kit for creating Facebook applications (.Net)
• Facebook.Net (.Net)
• fbasync (.Net)
• Javacript Client Library (Javascript)
• Official PHP Client Library (PHP)
• RFacebook (Ruby)
• FB4J (Java)
• JavaBook (Java)
• Facebook Java API (Java)
• PyFacebook (Python)
• minifb (Python)

Digg

• PHP Digg Toolkit (PHP)
• Net::Digg (Perl)
• Python Toolkit (Python)
• Dustin's Java Library (Java)
• Tim's Java Library (Java)
• Headzoo Java Wrapper (JAva)
• Digg API Wrapper (Ruby)
• diggr (Ruby)
• JavaScript API Toolkit (Javascript)
• Headzoo .NET Wrapper (.net)

Delicious

• PhpDelicious (PHP)
• Delicious API Wrapper (.net)
• NDelicious (.net)
• Delicious API (Java)
• Dishy (Javascript)

Flickr

• flickr api (ActionScript)
• Flashr (ActionScript)
• Flickr API Interfaces REST (ActionScript)
• as3 flickr lib (ActionScript)
• Flickcurl (C)
• CFlickr (Cold Fusion)
• Clickr (Common Lisp)
• Curlr (cUrl)
• dFlickr (Delphi)
• flickrj (Java)
• jickr (Java)
• Flickr.NET (.net)
• ObjectiveFlickr (Objective-C)
• Flickr::API 0.03 (Perl)
• Flickr::Upload 1.06 (Perl)
• PEAR::Flickr_API (PHP)
• phpFlickr (PHP)
• Phlickr (PHP%)
• Beej's Python Flickr API (Python)
• FlickrClient (Python)
• flickr.py (Python)
• REALflickr (REALbasic)
• flickr-ruby (Ruby)
• flickr.rb (Ruby)
• rflickr (Ruby)

Google Maps 

• GMaps Utility Library (JavaScript)
• PHP Google Maps API (PHP)
• PHP Wrapper (PHP)
• Google Maps .Net Control (.Net)
• C# Wrapper (.Net)
• pymaps (Python)

MySpace

• Official OpenSocial Javascript Library (Ruby)
• Official OpenSocial ActionScript Library (ActionScript)
• Official MySpace REST API PHP Library (PHP)
• Official MySpace REST API Ruby Library (Ruby)
• ActionScript2 Library
• OpenSocial Framework
• PHP Library
• Ruby Library
• Python Library
• VB.Net Library

Have I missed any? Post them below and I'll try to update the post from time to time