A ROM-Marketplace Addon for RetroPie where you can download and install ROMs.
wget https://github.com/pgmystery/retropie-marketplace/archive/master.zip -P /tmp/ && unzip /tmp/master.zip -d /tmp/ && python /tmp/retropie-marketplace-master/marketplace/setup.py && rm -rf master.zip && rm -rf retropie-marketplace-master