Free alternatives to Helvetica that actually work
Helvetica is the default answer to “what font should this be”, and licensing it properly for a web project or an app is not cheap. Several open-source fonts do the same job.
Inter
The closest thing to a modern Helvetica for interfaces. It was drawn for screens, so its letters are slightly wider and more open than Helvetica at small sizes, which makes it noticeably more legible in UI text. If you are choosing one replacement and not thinking further about it, choose this.
Roboto
Slightly narrower, with a few distinctive letters — the R leg and the k junction are its own. It is the most widely deployed sans on the web, which cuts both ways: universally available, and universally familiar.
Arimo
The pragmatic option, because it is metrically compatible with Arial. Every character occupies exactly the same width as its Arial counterpart, so substituting it will not reflow a layout. That matters when replacing a font in an existing document or template.
Archivo
A grotesque with more personality and a larger x-height. Worth considering when you want the Helvetica role — neutral, workmanlike — without the Helvetica neutrality.
What to actually compare
Do not compare individual letters side by side; that will always disappoint. Set a full paragraph in both at the size you will actually use, and look at the overall texture. Proportion and stroke weight determine whether a substitution reads correctly, far more than the shape of any single character.
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