In the self-promotion category, we'd like to mention some regularly-scheduled excursions from Palermo. These are escorted day tours to Segesta, Erice, Monreale (shown) and Cefalù. You can join them through concierges at a few participating hotels in central Palermo or online at Great Sicily Tours. Believe it or not, these are the only guaranteed, scheduled day tours from Palermo. Nobody else offers anything similar.
For customized ("tailor-made") walking tours in Palermo check out Jackie Alio's site, Palermo Guide.