I am playing Mafia 2 and I reached now chapter 9. I am now wondering if is there anything else to find in the city outside of what proposed by plot. I mean additional missions, unlockable, etc.
Short answer, not a lot.
Mafia II is about as linear as they come. It's not necessarily a bad thing, but it's an open world game, with no real openness. Other than what's part of the story, there are really only a few side quests that will net you a few achievements/trophies that you can win.
There are also 2 DLC packs that extend the story for some of the characters: Jimmy's Vendetta, and Joe's Adventures.