workers-chat-demo

You can persist data with their edge stores (see the sibling comment), or use it to transpose between different API schemas, or as an API gateway that fetches consolidates, consolidates, and caches multiple micro-endpoints, or use it to run Doom (https://blog.cloudflare.com/doom-multiplayer-workers/) or IRC (https://github.com/cloudflare/workers-chat-demo) a variety of smaller useful things (https://developers.cloudflare.com/workers/examples/) I am not affiliated with Cloudflare, but Workers is by far my favorite cloud/serverless platform -- so much simpler and faster than Lambda, etc. (if your use case is simple enough)

If you wanted something where you could get pretty far without spending any money, Cloudflare Workers + Durable Objects would be an interesting path. You would have to implement some of the functionality yourself, but there are some examples. Like this example chat app: https://github.com/cloudflare/workers-chat-demo/blob/master/... It supports private chat rooms, which is somewhat analogous to a pub sub topic.