The mission statement of Tesla Motors is to "accelerate the advent of sustainable transport," and it can be argued that the success of its Model S is doing this, at least to some extent, by inspiring other automakers to build long-range electric vehicles themselves. Witness the Porsche Mission E concept and Audi E-Tron Quattro Concept as two recently unveiled examples. Sharing the Supercharger system is another way to speed things up.
When EV owners of other marques want to take a trip, they may find themselves facing a hodge-podge of charging networks, each with slightly different standards, availability, and fees. Supercharging for free at well-mapped and easily accessible locations can only make things easier, and Musk has long said he would like other companies to make use of the network.
With some manufacturers declaring allegiance to the CHAdeMO and others, the SAE Combo system, it seemed like Tesla might not get any takers, but finally it may be getting traction. Musk mentioned the development on at least two different occasions recently at speaking engagements in Berlin: once in a morning discussion, and later during a larger meeting with the German Minister of Economy & Energy, Sigmar Gabriel.
While in the first instance, Musk used the plural "manufacturers," indicating there may be discussions with more than one firm, the second mention might be more relevant to the near term. In that case, while answering a question about sharing the Superchargers, he stated that "the CEO of one European car company, not a German car company, has approached us recently about doing exactly that, and we're super supportive of anyone who wants to do that."
The question now becomes, "who will be the first to use the Tesla high-speed network?" With the specific mention of a European company, and the exclusion of German ones, our best guess is Aston Martin. Its CEO Andy Palmer is quite bullish on electric vehicles and the iconic British brand already has a test mule for an 800-hp electric Rapide on the road, not to mention its fabulous DBX under development.
You can watch Musk makes these statements in the two videos included here. The first (below) is the short clip from a breakfast talk, while the second is the longer presentation with Sigmar Gabriel and includes an opening address by Musk, followed by questions from the audience (above). The Supercharger comment comes at 39:40. If you'd like to suggest a different automaker partner possibility, please do so in Comments.