The Australian Tax Office website has a wealth of material including all ATO Rulings. While not law and sometimes incorrect or incomplete (as evidenced in Court decisions) they are a useful place to get a “feel” for how something may be viewed by the ATO.
The Commonwealth Treasury tax treaties page is a useful source for up to date versions of tax treaties and explanatory material.
ComLaw (Commonwealth Law) is the official website setting out all Commonwealth legislation, including the legislative history of current and superseded Acts of the Commonwealth Parliament.
The Tax Institute has useful information for tax professionals.
BarNet Jade has a searchable database of Australian legal decisions.
The Australasian Legal Information Institute has a large database of legislation, cases and legal articles.
The HMRC website has a lot of basic material on UK income tax, capital gains tax and inheritance tax.
The US Internal Revenue Service has more than you could ever want to know about the US Internal Revenue Code and US estate tax