Look at diagrams.net (formerly draw.io). It is gratis (free of cost). I use it intensively as a software engineer to architect and design different workflows.
There is also a desktop app for the same web tool.
Before you try MS Visio, have a look at yEd. It's free, works on Windows as well and can also draw flowcharts.
You can adjust the colors etc. to make it look closer to Visio.
Downside: I needed to get used to its usability. It doesn't behave like I expected it would. Usually I accidentally draw a lot of more boxes than needed.
ALL FREE TOOLS:
I like yEd from https://www.yWorks.com (If you like to move your mouse/cursor).
To render DLS I use http://blockdiag.com/en/actdiag/index.html
(PlantUML is complexer: it allows more complexity in your diagrams, but you have to write more code to get the same diagram rendered).
I think this page is very usefull for questions like this:
draw.io can be used to draw flowcharts among other schematics and drawings. If you want an offline version, you can download the app at https://about.draw.io/integrations/.
I have used both the online and the offline versions for flowcharts on OSX and I can say they work quite well.