In the United States, the Supreme Court does not make a decision about the validity of a law without a specific case presented to them. Are there any examples of common law systems in which a court is able to make a decision without actually having a case at hand?
Yes. In some common law jurisdictions, such as Canada, the United Kingdom and even some U.S. states, the government may under some circumstances refer a legal question to the appropriate Supreme Court (Privy Council in the U.K.) for an advisory opinion. These opinions are non-binding, but have large influence because they are often made by the same judges that would otherwise end up dealing with the question should it occur in a case.
One common law country in particular stands out in this field: Ireland. Under Article 26 of the Irish Constitution the President may, with some exceptions, refer a bill to the Supreme Court to test its constitutionality. The referral is optional, but once made, the Supreme Court's decision is binding. The relevant portion:
3 1° In every case in which the Supreme Court decides that any provision of a Bill the subject of a reference to the Supreme Court under this Article is repugnant to this Constitution or to any provision thereof, the President shall decline to sign such Bill.
3° In every other case the President shall sign the Bill as soon as may be after the date on which the decision of the Supreme Court shall have been pronounced.
This power was last used successfully in 2004.