Like the Reformed Presbyterian Church in North America, the Free Church of Scotland has historically allowed only unaccompanied singing of Psalms in the liturgy. However, its synodical assembly has now decided, by a narrow majority, to permit extrabiblical hymns and instruments in worship for those congregations desiring it. Given that the assembly was divided on the issue, many are unhappy with the decision — with one minister considering leaving the "Wee Free" for another Reformed denomination — thereby incurring the scorn of at least one member of the press. The Free Church's statement can be found here.
Incidentally, I have just been lent a copy of the RPCNA's new Book of Psalms for Worship, which replaces the Book of Psalms for Singing. I have not yet had a chance to look at it carefully, but at first glance I see that it is strictly limited to the 150 canonical Psalms, excluding other biblical material, such as the Decalogue, the Song of Hannah and the three Lukan canticles.