We rolled out the Amazon Simple Queue Service yesterday.
Using this service, any AWS user can create up to 4,096 queues. Each user is limited to a total of 4,000 queue entries (thanks, Greg Linden, for noticing that I got this wrong originally).
The queue will retain the data for up to one month. Each item in the queue can be up to 1MB in length (non-text data must be encoded, of course).
The queue is a fundamental building block for distributed applications. The queue implements the classic FIFO (First-In, First-Out) programming model.