There are several good answers here already (Google PubSub, AWS SQS, etc).
I'll just add that you can do pub/sub through Redis as well. i.e. don't make your servers pub/sub to eachother (e.g. what if one crashes, needs to be updated, etc)... instead, make them pub/sub to a Redis instance (or AWS Elastic Cache if you don't want to be tied down to AWS SQS).