Are there any operating SSTO (single stage to orbit) vehicles? Are there others being tested such as Reaction Engine's Skylon?
Under development? Maybe. Define under development. Someone is always working on an SSTO, but none have ever delivered (not to say it is impossible, just that the track record to date is null). Is Skylon seriously under development? Hard to say. People WANT to develop it, but the question is, do they have the needed funding. As of now, no. Maybe in the future.
Several rocket stages have been shown to possibly be capable of SSTO performance numbers. (Saturn V second stage, maybe an Atlas without sustainer engines). But generally the payload delivered would be miniscule, or they have some other limitation.