Opensource Messaging Queues?

SuperID asks: "I just attended a very long, mostly boring briefing smattered with zillions of animated color power point slides touting "messaging middleware" as the salvation of scalable architectures. While the buzzword density was extremely high ( "Data Mining" and "OLAP for the Information Warrior" *shudder* ) I can at least recognize a trend when it smacks me in the face. So, are there Opensource equivalents to products like IBM MQSeries or Microsoft Message Queing Server? Are there open standards that define how these types of servers operate? Maybe an Internet RFC?"