I have read a lot about Skylon which is a single stage to orbit concept space plane with a SABRE engine designed by the UK think tank Reaction Engines limited. However, in recent months it seems like the project has stalled. Last year, the key engineer behind the project, Alan Bond, retired which seems to have led to a reduction in activity and media attention for it.
I think the SABRE engine and Skylon project is also interesting because it could potentially provide a concept for a supersonic high altitude airliner with zero emissions if powered by hydrogen. Reaction engines developed the A2 as a result of the research work done on the Skylon project.
Also, Elon Musk has talked recently about an electric supersonic VTOL aircraft which could fly at very high altitude where there is little air resistance. However, it still seems battery energy density is not sufficient to allow this. Such an aircraft would have to be powered by liquid hydrogen or methane in its launch stage like the Skylon and then land. If it were VTOL it would not need the 6km runway which is required by the Skylon.