SimplyFortran is a “modern Fortran development environment” from Approximatrix LLC. Features include “the GNU Fortran compiler, an advanced development environment including project management, and an integrated debugger for fast and easy installation”, along with “impeccable” support for the FORTRAN 77 standard.
SimplyFortran 3.9 is priced at $99 for an individual license for macOS 10.15, Windows or Linux, with a 30-day trial period, or $159 for an individual license covering all three platforms. Older versions are available for earlier systems with Version 2.41 working on Macs back to Mac OS X 10.6. The latest version “includes improvements to the development environment and important bug fixes on all platforms.”