Besides the maps for the metro, bus and train systems, it's convenient to have a good map of the city. You can ask for a free and very detailed map at the tourist information offices. If you aren't planning visiting them, you can buy a street map at many street kiosks.
You can also download from GoBarcelona the following maps in PDF format:
This map covers the zones of Sants (including the Sants Station), Left Eixample and Montjuic.
This map covers the zones of Gràcia, Right Eixample, Ciutat Vella, Barri Gòtic and Barceloneta.
This map covers Poblenou, Villa Olímpica and La Sagrera.
This map shows a general, although low detail, view of Barcelona.
This map shows a detailed view of Barri Gòtic, part of Ciutat Vella, and the area surrounding Plaça Catalunya.