Services provides access to core functionality to remote clients (things like nodes, users, taxonomy, etc.) The following projects integrate with Services to provide the same functionality outside of what is included in core.

D8+

D7

• Accessible Content (6.x only, for 7.x use http://drupal.org/project/accessibility)
• Content lock
• Deploy
• DrupalGap
• Drush Webservice
• Flag
• Graphmind
• Hosting Services for Aegir
• Mast
• Media Front
• Nodequeue
• OG Vocabulary
• RDF (6.x only, for 7.x use https://www.drupal.org/project/rdfx and look at RDF module in core)
• User Relationships
• User Points
• Services Address Field
• Services Contact
• Services Guzzle
• Services Entity
• Services Views
• Services client
• Services Rules
• Services search
• Services Search API
• Backbone
• ngApp

D6

• Accessible Content (6.x only, for 7.x use http://drupal.org/project/accessibility)
• Blue Droplet Video (6.x only)
• Content Distribution (6.x only)
• Country Codes API (6.x only)
• Dash Media Player (6.x only)
• ERP (6.x only)
• Magento (6.x only)
• RDF (6.x only, for 7.x use https://www.drupal.org/project/rdfx and look at RDF module in core)
• Solr Search Service (6.x only)
• XMPP Framework (6.x only)

There are also some modules that provide Services for external data or other processes.

• Google Analytics Service