To install, take a build of the ArchivesSpace master branch (e.g. http://aspace.hudmol.com/build-snapshots/archivesspace.zip). Then:
unzip -x archivesspace.zip
cd archivesspace/plugins
git clone https://github.com/marktriggs/generate-accession-identifiers-plugin
cd ../
echo "AppConfig[:plugins] = ['generate-accession-identifiers-plugin']" > config/config.rb
./archivesspace.sh
Browsing to the "New Accession" form should generate an identifier
like 2013 000
.