ar\connect\db Database connectivity, based on PHP's PDO.
ar\connect\ftp FTP client API.
ar\connect\oauth OpenAuth client API. Based on PHP's OAuth pecl extension.
ar\connect\rss RSS Client API.
ar\connect\soap Simple SOAP API for clients and servers. Based on PHP's SoapClient and SoapServer.
ar\connect\twitter API to connect to twitter, based on the PEAR twitter API. Requires the PEAR twitter library.
ar\connect\xmlrpc API to connect to xml-rpc servers or create xml-rpc servers. Based on Ripcord.

In addition you can use ar_http::client() to do HTTP requests.