In an idealistic world, I'd take the wallet (no point leaving it on the street so someone can nab it) and I'd hand it into the nearest police station. It's not that I couldn't deal with a guilty conscience, it's just that I believe that the good outcome should be the ultimate outcome. Also, in Ireland, if someone hands something into the police, that person can take that thing as his/her own if it hasn't been claimed by the rightful owner within 366 days. So really, handing it in doesn't necessarily mean I'd lose it.
That said, it does depend on where I was headed at the time. If I was going to some sort of art exhibition, yeah, I might take the wallet and see if there were some good paintings for sale, but I'd probably end up giving what's left to the police anyway.