I'm impressed with Ansible. There are fewer concepts to learn to get up and running (compared with similar tools I've used).
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I've been impressed with code written by Fable in my testing:
Difftastic: found small optimisations in a hot loop I'd already profiled extensively. Helped me prototype Dijkstra to A* too (hard to find a good heuristic).
Garden: Found some real bugs in my simplistic typechecker.
I'm impressed and slightly surprised that I can browse the web even when my local library's wifi blocks detectportal.firefox.com ("Download sites are banned").
It seems that Firefox only requires the website to be redirected when there's a captive portal.
I'm super impressed by how many lint rules can be implemented with just eslint's no-restricted-syntax and selectors. Example:
{
selector: "ExportNamedDeclaration[declaration=null][source=null]",
message: "Add `export` directly to the function instead of a block export."
}