Google's Roboto is a very close drop-in replacement for Helvetica. Almost the same metrics, slightly more "condensed"/DIN feel.
You may also consider FreeSans (GNU replacement for Helvetica) and Liberation Sans (which is a metric replacement for Arial, which is an alternative for Helvetica...)
Roboto to me feels very 'edgy' and active, and less neutral than Helvetica.
Most Linux systems come with the Nimbus fonts as part of the printer drivers, and Nimbus Sans is very close, but apparently (AFAIK) only has a limited character set unless you pay for the full version. GNU FreeFont is practically the same as Helvetica and Nimbus but gives you a much larger character set for free.
Liberation is not quite as geometric as Helvetica/FreeFont/Nimbus.
Here's a comparison of a few fonts (the first is San Francisco by Apple).
The 'r' and 't' in Liberation look quite a bit different than the first three, which are basically Helevetica clones. The ring in the 'b' also has a weird lower-left side. It's like 20% of the way to being a Serif font.
I haven't used MS fonts in a while but DejaVu reminds me very much of Arial, and to me it looks a lot worse. Especially the way the 'S' looks is quite awkward.
I just don't like the spacing with DejaVu... it's awkward is about all I can say about it. I do like Roboto a lot myself, and it's generally my default these days.
Open Sans is a poor replacement. I quite like Source Sans Pro but it isn't the same as Helvetica