Today I saw some disturbing news on the Internet:

• https://twitter.com/ReinH/status/1098663375985229825
• https://www.reddit.com/r/devops/comments/at3oyq/it_looks_like_ibera_is_gutting_travis_ci_just_a/
• https://twitter.com/hynek/status/1098908925091610624

TL;DR Idera (new Travis' overlord) has unexpectedly fired an enormous amount of engineers. There are rumors/suggestion that Travis CI platform itself may be trashed.

With that in mind, there are several candidates:

• Azure DevOps (former Pipelines) which AFAIR allow up to 10 simultaneous jobs running and I've got positive feedbacks from various FOSS maintainers included but not limited to tox, virtualenv, cpython etc. Supports GNU/Linux, win and macOS.
• Circle CI has 4 simultaneous jobs for GNU/Linux (container-based only) and supports non-parallelized macOS (VM) per request.
• The most popular CI for testing Windows-based projects, recent year they've also added GNU/Linux support. Jobs are moderately throttled.
• GitLab CI, supports integration with GitHub but only 1 year or free trial for that. Mostly GNU/Linux container based unless you bring your own nodes (which implies a need for additional costs).
• Shippable, supports mostly GNU/Linux unless you BYON. Hugely throttled if free.
• GitHub Actions. Now it limited public Beta but we have access. It's container-based with a limited runtime. For example, you cannot spawn other containers from within those. And you probably can't spawn VMs either. But we can offload linters and probably unit tests there.
• Zuul (as proposed by @pabelanger)
• smth else I haven't been playing with

• there's an extra option which is basically using all of them and load-balancing jobs across multiple CIs which helps to overcome throttling a bit