I tend to use TextMate on Mac or SublimeText on other platforms. Fortran also is usually very close to C, and most standard debuggers like ten or lldb (even when LLVM had no Fortran front-end) can be used without problems. It’s a bit barebones compared to recent IDEs, sure. What I would like in particular is better refactoring and static analysis tools (though the lack of the latter is mitigated by the strong type system and some very good compilers like NAG). There is no real equivalent to RStudio or Matlab.