To add to the post in Nigel's own arrangements thread regarding Villanelle and the fact that its composer (Paul Dukas) went out of copyright at the beginning of this year, there are other composers where the majority (I'll explain that bit in a minute) of their works passed into the public domain on January 1st 2006. Notably for brass players, Victor Ewald is one of them (he who wrote or had transcribed 4 brass quintets, the first of which is also known as Symphony for Brass).
Also, Alban Berg, Josef Suk, Mikhail Ippolitov-Ivanov (Caucasian Sketches) and Johan Halvorsen (Entry of the Boyars) died in 1935.
To explain my earlier 'majority' comment: Some of the works of certain composers who died more than 70 years ago still enjoy copyright protection by dint of things like posthumous publication/performance of the original works, or those parts of a work, that were completed by another hand (Mahler's 10th Symphony, completed by Professor Deryck Cooke) for instance. I do believe there are some Alban Berg works which are still protected for reasons of posthumous publication, though I wouldn't swear to it. Particularly if a composer has just 'gone out of copyright' it is, in my view (as a former employee of the PRS for 18 years) best to double check with the PRS if wishing to arrange the appropriate work/s.
Out of copyright composers
Out of copyright composers
Conductor, Arran Brass. Brass Band tutor for the Workers' Music Association Summer School. Founder and director, Arran Brass Monkeys Quintet.