For anyone interested on this subject while visiting Sicily. Guess it could be argued that the whole island is one big mafia sight.
Palermo:
On the A29 just before you really get into Palermo and just before the exit to Capaci is the memorial to Giovanne Falcone and the others that died in the bombing ordered by Toto Riina.Amazing the stretch of autostarda that was blown up.
The Delle Palme hotel on Via Roma where the 1957 drug conference took place, Lucky Luciano lived for years while exiled from America and the Baron Giusseppe Di Stefano lived for 50 years in exile under a mafia death sentence for messin around with a mafia wife.
The Piazza Marina where American NY cop Joe Petrosino was killed in 1909.
Montelepre - Where the Salvatore Giuliano story took place. Go to the Castello Giuliano restaurant and you'll meet his nephew Giusseppe, see some Giulianno memoribilia and can ask for a tour of the house he grew up in. Food was pretty good too!
Corleone - Besides the obvious Godfather thing....Toto Riina's hometown. A bit of a rough drive to get to it.
Castellamare Del Golfo - The town where Salvatore Maranzano was from.(The guy who started the American Mafia in the way we know it).
Those are just the ones I visited. For a good reference on this subject try Cosa Nostra by John Dickie.