Drupal user keep been developing a system which allows a Drupal website to act as an authentication server and allow client websites to be connected to it. Once you register an account on one of the websites in the network you can use those credentials to gain access to any other website there. There are a lot of other functions such as login, authenticate,file posting,node content posting, comment posting and a lot. A lot of drupal techniques exist for implementing these thing. I will introduce them in my blog.
1)xmlrpc
XML-RPC is a very simple RPC service. It is also possible to use XML-RPC within Drupal as a client but, again, this is in no way database-related: the client and server have to agree on predefined functions which will be made available remotely. Typically, these are at a higher abstraction level than database access, and have to implement access control, since Drupal does not use any form of in-database access control after the connection: all code can access all tables in the Drupal DB. Once you installed this module on your server, it will be available to any XML-RPC client, at the endpoint (example): http://www.example.com/xmlrpc.php. I will like to introduce "whereis" module of drupal for visitors if not familiar:-
http://drupal.org/project/whereis
This is an application that runs on your Symbian phone (Nokia S60), queries your GPS for your lat/long, and sends your coordinates to Drupal using XMLRPC.
At the moment, Drupal will receive a node id, latitude and longitude, and it will simply update the location-enabled node with the new coordinates.
drupal hook (hook_xmlrpc) :-
http://api.drupal.org/api/HEAD/function/hook_xmlrpc
2)services
The services module exists to allow other services to communicate with Drupal but there is no easy way for Drupal to tell other services information about its own content.There are a number of services which would be beneficial for Drupal to implement. For drupal 7 services modules is going to be in core. This would allow use of XML-RPC, JSON, RSS, REST, SOAP and more to be loaded through a common Services API.