XML-RPC API that supersedes the legacy Blogger, MovableType, and metaWeblog APIs. Some clients also exist for different programming languages.

Components

Posts (for posts, pages, and custom post types) - Added in WordPress 3.4
- wp.getPost
- wp.getPosts
- wp.newPost
- wp.editPost
- wp.deletePost
- wp.getPostType
- wp.getPostTypes
- wp.getPostFormats
- wp.getPostStatusList
Taxonomies (for categories, tags, and custom taxonomies) - Added in WordPress 3.4
- wp.getTaxonomy
- wp.getTaxonomies
- wp.getTerm
- wp.getTerms
- wp.newTerm
- wp.editTerm
- wp.deleteTerm
Media - Added in WordPress 3.1
- wp.getMediaItem
- wp.getMediaLibrary
- wp.uploadFile
Comments - Added in WordPress 2.7
- wp.getCommentCount
- wp.getComment
- wp.getComments
- wp.newComment
- wp.editComment
- wp.deleteComment
- wp.getCommentStatusList
Options - Added in WordPress 2.6
- wp.getOptions
- wp.setOptions
Users
- wp.getUsersBlogs
- wp.getUser (3.5)
- wp.getUsers (3.5)
- wp.getProfile (3.5)
- wp.editProfile (3.5)
- wp.getAuthors

Obsolete Components

Categories - use Taxonomies instead, with taxonomy='category'
- wp.getCategories
- wp.suggestCategories
- wp.newCategory
- wp.deleteCategory
Tags - use Taxonomies instead, with taxonomy='post_tag'
- wp.getTags
Pages - use Posts instead, with post_type='page'
- wp.getPage
- wp.getPages
- wp.getPageList
- wp.newPage
- wp.editPage
- wp.deletePage
- wp.getPageStatusList
- wp.getPageTemplates

rubypress WordPress XML-RPC client for Ruby projects. Mirrors this documentation closely, full test suite built in.
wordpress-xmlrpc-client PHP client with full test suite. This library implement WordPress API closely to this documentation.
WordPressSharp - XML-RPC Client for C#.net
plugins/jetpack Related: Jetpack by WordPress.com enables a JSON API for sites that run the plugin
plugins/json-api Related : WordPress JSON api