Plenty o'Instagrammers love the old vignette. I've been known to use it myself. However, you'd generally rather have control of it than have to deal with it. Decent software like Lightroom will allow you to apply lens-specific profiles to deal with all the optical distortions introduced by the hardware.
I don't know any of the free stuff, unless you're talking phones (I use Snapseed). It reflects my bias, no doubt: I believe photo software should make it easier to get what you shot so that you can spend more time shooting. Thus, I never made more than a half-hearted attempt at doing any processing in Photoshop. Lightroom I can process a month's worth of photos in half an hour. Anything needing more hard-core tweakitude than Lightroom can do, I throw at creative kit.
Although, Lightroom is now shit on a stick and apparently Macphun saw enough weakness that they're now making Luminar for PC. I might go that way myself.