Everyone's been giving you names of fonts that look similar to Helvetica (some more than others) but depending on what kind of project it is that you're working on you might not need any of those.
Unless you need something that looks almost exactly like Helvetica, Arial might be an option you should consider. It comes pre-installed in (almost?) every Windows version and also Apple devices. Unfortunately Arial Narrow isn't included in iOS but, since both Helvetica and Helvetica Neue come pre-installed in all Apple devices, you wouldn't have to worry too much about it.
Your font stack could be something like
font-family: "Helvetica Neue", Helvetica, Arial, sans-serif; and you'd be covering the most common devices and operating systems (except for Android which I think doesn't come with any of those two font families).
Keep in mind this is far from ideal in most cases but would allow you to avoid licensing costs and reduce the time it takes for your website to load (since it will not be downloading fonts). This will also take some load off you server even though serving fonts isn't such a big deal.
Here's a link to a site with some of the fonts that come pre-installed with different operating systems: http://www.cssfontstack.com. And here's another one to a Wikipedia article about the usage share of the different operating systems: https://en.wikipedia.org/wiki/Usage_share_of_operating_systems.
EDIT: This site is great when you wanna check what font would be used in each browser and operating system: http://fontfamily.io/
font-family: "Helvetica Neue"It's legal because I'm not serving the font myself; instead the browser is using it from the user's system if it exists.