You could consider CD tools as being the natural evolution of the aforementioned CI tooling. Instead of controlling and orchestrating the build and test process, CD tools control and orchestrate the act of deploying the built software components to the various environments you have. Currently, there doesn't seem to be a standard set of standalone CD tools. Instead, many vendors and tool creators have extended their products to include this functionally.
Before committing to a tool or solution, you should consider some of the following questions: