As its name implies it is a Fortran 77 compiler. Its original author was James Craig Burley. The compiler back end is the GNU Compiler Collection backend, i.e. the same as used by GNU C, g++, Objective-C etc.
G77 has largely been replaced by the gfortran compiler as of GCC version 4. gfortran is not a Fortran 77 compiler though and does not entirely replace g77 as far as Fortran source compatibility is concerned.
A historic version of the g77 home page has been archived here.