List of things that let you write a single program which performs a distributed task

These are problem areas where you'd normally use multiple separate programs/configurations/whatevers. But with these things, you can use a single program instead.

Distributed languages and distributed DSLs.
Distributed programs execute pieces of their code in parallel on different processors. [...] The first requirement for distributed programming support is therefore the ability to assign different parts of a program to be run on different processors.
– Section 1.2, Programming Languages for Distributed Computing Systems

See also the power of libraries over servers.
Software-defined networking (SDN). Note that the SDN term is somewhat abused, and today lots of people use it to mean "overlay networks implemented in software". I mean the original sense:
An SDN consolidates the control plane, so that a single software control program controls multiple data-plane elements. The SDN control plane exercises direct control over the state in the network’s data-plane elements (i.e., routers, switches, and other middleboxes).
– Section 1, The Road to SDN: An Intellectual History of Programmable Networks
Liveviews, sometimes called "HTML over the wire". I think this is best explained by the docs for replica:
In contrast to traditional virtual DOM implementations, a remote virtual DOM runs on the server and is replicated to the client, which just acts as a dumb terminal. [...] With a remote VDOM, all code runs on the backend – no need to come up with ways to share functionality between server and client, to validate forms twice or authenticate API requests.
– Remote virtual DOM, replica: A remote virtual DOM library for Haskell
Capability systems let you control access by passing around objects in a single conceptual program, instead of many separate ACL configurations.
Single-program systems. :)

Problem areas where you'd normally write a single program, but with so-and-so tools, you can instead write multiple programs (for locality and performance, usually) are also interesting (transforming a shell script using ssh into multiple communicating programs; BPF; XDP; the storage equivalent of XDP; uploading code into a service in general; etc.), but not eligible for this list since they're the exact opposite.