Nimbus Sans
Designed by the Urw Foundry type, Nimbus Sans is a great Helvetica substitute.

Nimbus Sans is based directly on Helvetica, and you can see that it is suitable even in features that are not owned by other fonts on this page - the shape of the letters ends in the horizontal and vertical terminals, the "R" legs have an elegant bend for it, "G" has a little spurs down, etc. One big change: darker - which, in the world of typography, means the shape of the letters is thicker at a certain size, and therefore looks a little darker when you narrow in the large block of the text. You might consider increasing high lines or alleviating the color of the text to compensate, but it's up to you.

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Roboto
The popular default Android font, Roboto, is open-source and free for use.

Roboto's lowercase shape is slightly thinner than Helvetica. As similar, it's not perfect, but Roboto is optimized for the screen. It might feel too familiar to users if used in Android applications, but it is highly recommended for other uses 👍. Because the shape of the letters is a little thinner and therefore more compact, I feel this is working best for a busy and important UI where space is essence and cannot be wasted.

Tex Gyre Heros
Tex Gyre Heros includes some improvements in Helvetica, but basically is deadringer.

This one is based on Nimbus Sans, which in turn is based on Helvetica. And you know what? This is basically dead. As long as you only need two burdens, it makes you closed.