We present Slipstream, a userspace solution for transparently selecting efficient local transports in distributed applications written to use TCP/IP, when such applications communicate between local processes. Slipstream is easy to deploy because it is language-agnostic, automatic, and transparent. Our design in particular (1) requires no changes to the kernel or applications, (2) correctly identifies (and optimizes) pairs of communicating local endpoints, without knowledge of routing performed locally or by the network, and (3) imposes little or no overhead when optimization is not possible, including communication with parties not using our technology. Slipstream is sufficiently general that it can not only optimize traffic between local applications, but can also be used between Docker containers residing on the same host. Our results show that Slipstream significantly improves throughput and latency, 16-100% faster throughput for server applications (and 100-200% with Docker), while imposing an overhead of around 1-3% when not in use. Overall, Slipstream enables programmers to write simpler code using TCP/IP "everywhere" and yet obtain the significant benefits of faster local transports whenever available.