WDDX

Simple data enconding format, handles scalars, arrays/structures, date and time. [http://www.openwddx.org/]

CML

A very complete format specification for chemical data: structural information (coordinates, sequence, etc.) and properties (charges, spectroscopical data, etc.). One of the first XML applications designed. [http://www.xml-cml.org/]

RSS

Widely used for news syndication (e.g. Slashdot, Freshmeat, PHPBuilder). [http://groups.yahoo.com/group/rss-dev/files/namespace.html]

XML-RPC

Provides a simple remote procedure call protocol and encoding specification. You can read and understand it over a cup of coffee (or tea or whatever is your favorite poison). Uses HTTP as its primary transport. [http://www.xmlrpc.org/]

SOAP

A heavier and more flexible specification for performing remote procedure calls, it includes specifications for transports, envelope handling, call redirection, data encoding, etc. It has as partners in crime the WSDL and UDDI specs. [http://www.w3.org/TR/SOAP/]