Since Diameter is a relatively new protocol introduced to support data in telecommunications networks, I think it's worth providing a series of links to the industry standards bodies that play a significant role in Diameter's development and in emerging use cases. This is not a finite list, just a good start. 

3GPP – The 3rd Generation Partnership Project

The 3rd Generation Partnership Project (3GPP) unites six telecommunications standards bodies, known as “Organizational Partners” and provides their members with a stable environment to produce the highly successful Reports and Specifications that define 3GPP technologies.

http://www.3gpp.org/

3GPP2 – The Third Generation Partnership Project 2

3GPP2 was born out of the International Telecommunication Union’s (ITU) International Mobile Telecommunications “IMT-2000″ initiative, covering high speed, broadband, and Internet Protocol (IP)-based mobile systems featuring network-to-network interconnection, feature/service transparency, global roaming and seamless services independent of location. IMT-2000 is intended to bring high-quality mobile multimedia telecommunications to a worldwide mass market by achieving the goals of increasing the speed and ease of wireless communications, responding to the problems faced by the increased demand to pass data via telecommunications, and providing “anytime, anywhere” services.

http://www.3gpp2.org/

MSF – MultiService Forum

The MSF is a global association of service providers, system suppliers and test equipment vendors committed to developing and promoting open-architecture, multiservice Next Generation Networks. Founded in 1998, the MSF is an open-membership organization comprised of the world’s leading telecommunications companies. The MSF’s activities include developing Implementation Agreements, promoting worldwide compatibility and interoperability of network elements, and encouraging input to appropriate national and international standards bodies.

http://www.msforum.org/

MSF_VoLTE _2011_WhitePaper

OMA – Open Mobile Alliance

OMA is the focal point for the development of mobile service enabler specifications, which support the creation of interoperable end-to-end mobile services. OMA drives service enabler architectures and open enabler interfaces that are independent of the underlying wireless networks and platforms. OMA creates interoperable mobile data service enablers that work across devices, service providers, operators, networks, and geographies. Toward that end, OMA will develop test specifications, encourage third party tool development, and conduct test activities that allow vendors to test their implementations.

http://www.openmobilealliance.org/

ETSI – The European Telecommunications Standards Institute

ETSI produces globally-applicable standards for Information and Communications Technologies (ICT), including fixed, mobile, radio, converged, broadcast and internet technologies. ETSI is a not-for-profit organization with more than 700 ETSI member organizations drawn from 62 countries across 5 continents worldwide.

http://www.etsi.org/WebSite/homepage.aspx

TISPAN – Telecom & Internet Converged Services & Protocols for Advanced Networks

Since its creation in 2003, ETSI TISPAN (Telecommunications and Internet converged Services and Protocols for Advanced Networking) has been the key standardization body in creating the Next Generation Networks (NGN) specifications

http://www.etsi.org/tispan/

ITU – International Telecommunication Union

ITU is the United Nations specialized agency for information and communication technologies – ICTs. We allocate global radio spectrum and satellite orbits, develop the technical standards that ensure networks and technologies seamlessly interconnect, and strive to improve access to ICTs to underserved communities worldwide.

http://www.itu.int/en/pages/default.aspx