Useful pressure systems web links evaluated and selected by a network of subject specialists are listed below:

ASME Pressure vessels and piping division
Details of ASME pressure vessels publications 

The Carriage of Dangerous Goods and Use of Transportable Pressure Equipment Regulations 2004 

BERR Pressure Equipment Directive 97/23/EC 

The British Standards Institution (BSI)

IMechE Pressure Systems Group
Includes information on events and the PSG Newsletter

National Board of Boiler and Pressure Vessel Inspectors (USA)
This US site gives access to information about the Board, the full-text of its Bulletin and other publications aimed at those working with boilers and pressure vessels. Some full-text technical articles are also available. Guidelines on stamp and mark requirements are provided.

Pressure Equipment Directive: overview
Containing essential reference information in order to design, produce, market and put into service Pressure Equipment and Pressure Assemblies in Europe

Pressure Vessel Research Council
The site gives information about the council and its activities. In addition, abstracts of all the Bulletins are provided. 

Process Engineering Directory

Process and Packaging Machinery Association
Includes a worldwide supplier directory 

United Kingdom Accreditation Service
Accreditation body for testing and calibration laboratories, certification and inspection bodies