Postcrossing is a fun pastime I've been playing around with again as of late. It's rather simple: you get an address and a person's profile to send a (real-life, physical) postcard to, and when it arrives, the system assigns your address and profile to someone else. It's enjoyable to pick a card and tailor the message to the recipient's interests, you get to use up your stickers and ink stamps, and there's something very human about the whole thing.