Patent Office Websites
The European Patent Office has an up to date version of the European Patent Convention, with all the associated Rules and Protocols. The Guidelines for Examination are an indispensable tool for dealing with applications before the EPO. The case law of the Boards of Appeal can be found via a search interface and in a regularly updated book (the latest version of which was published in July 2016). The Official Journal, which is published monthly, contains updates to the law in the form of decisions by the Administrative Council and other announcements such as decisions and notices from the EPO President.
The EP Register and Espacenet are the services most commonly used during day to day practice, with the Register providing a constantly updated status and prosecution file for all published EP applications and Espacenet providing a comprehensive database of published application. The European Publication Server can sometimes be useful when searching for something specific, like patents that have been subject to the limitation procedure (which is where I got the information for this post).
Another database that is often useful is the register of European patent attorneys. You can also find out here who has passed the qualifying exams, which is of course not necessarily the same thing.
Patent Cooperation Treaty (PCT), the most important being the Treaty and associated Rules, the Applicant's Guide, an updated list of the Contracting States and a list of the time limits for entering the national/regional phase. There is also a useful calculator for time limits relating to PCT applications.
WIPO also provide a useful directory of national IP offices.
US Patent and Trademark Office has lots of information about US patent law but is horrible to navigate. Most day to day activities will generally involve using the Patent Application Information Retrieval (PAIR) file inspection service and the Manual of Patent Examining Procedure (MPEP). The USPTO also provide consolidated versions of the relevant legislation. Various searching tools for US patents and applications are provided, but these are generally not as useful as Espacenet.
legislation.gov.uk, which contains updated versions of Acts, but is not up to date with everything and can be a bit difficult to navigate.
The British and Irish Legal Information Institute (BAILII) is a good source of published case law from the UK, such as those from IPEC, the Patents Court, the Court of Appeal and the Supreme Court.
Companies House is a useful source of official information on registered companies, which I always check when starting with a new client to make sure the correct information is on file.
UK Patent Law
I run a wiki-based website called ukpatents, which contains an updated version of the Patents Act 1977 and the Patents Rules 2007 and is cross-referenced to make it easy to navigate between different sections and rules.
European Patent Law
XEPC, which is a cross-referenced version of the EPC and the associated Rules, Guidelines and Case Law. This can be a useful starting point for researching questions relating to particular provisions of the EPC.