I'm all about the found (or free) souvenir — a pen here, bottle of lotion there. Half the time though, I only end up with the pen since I've tossed it in my purse sometime during the trip, and the sweet potion lasts for like one more week after I get home again. However, another thing I can't seem to leave without is a key card. It's rarely on purpose, just something I often dig out of my wallet after vacations are done.
Then I spotted a recent article saying that the electronic card is a great remembrance, especially in case your personal information is somehow stored on it. One time, I intentionally kept a card to pass along to a pal who lived in the city (since the bar made guests flash a key to get in), but that was my only intentional keepsake, albeit not for myself.