Lately I was pointed to http://opencpu.org/ . Nifty website, but after browsing for a little while I wasn't so sure where it is located in the R landscape compared to e.g. rApache or RPy2.
After waiting a long time for the server to come back I was finally able to read the architecture section, but that wasn't too comprehensive. I'm looking for a more detailed explanation of what OpenCPU is, how it is intended to be used, and how this compares with existing tools such as rApache and RPy2.
In a nutshell: OpenCPU is a layer on top of the regular tools (e.g. RApache, rpy2) that defines a framework and protocol for interacting with R. It handles stuff like object serialization, security, resource control, reproducibility etc, while abstracting away technicalities.
This paper on arxiv goes into more detail on the motivation and design of the system and API.