When I was at UC Berkeley, there was a seminar of this sort called "Many Cheerful Facts" (see here if you want to know why). It was organized by graduate student. Every week, one student would be recruited to speak and another would be recruited to bring baked goods, and then everyone ate the baked goods plus whatever lunch they brought and enjoyed the talk. The talks were usually expository about some major result or concept. It was a lot of fun!