Many sans serif fonts are labeled as similar to Helvetica. But even the fonts that are fairly similar to Helvetica have some significant difference, especially if they are available for free. That is, in part, to prevent the creators of those copy-cat fonts from copyright questions. Depending on the style of your project, looking through a database of sans serif fonts may provide you with an ideal match. You can look for certain key words that tell you a designer was thinking of a Helvetica design, such as Swiss or Helv. The names can be particularly subtle: Geneva has been used in the past to reference Helvetica's Swiss origins.