I need a compiler for Fortran 77 in linux.
Are there any free compilers out there that people use?
I've heard about g77, but I can't find the rpm or how to install it in linux.
GCC's Fortran compiler name has changed: g77 has been replaced by gfortran, which handles Fortran 95 and possibly more recent variants.
The package in Ubuntu is called gfortran:
sudo apt-get install gfortran
(or use synaptic)
The package in Fedora is called gfortran and is part of gcc, which you probably already have:
sudo yum install gcc
(or use pirut)
Similar searches should work for your distribution as well.
Update: On RedHat 4.1.2 the package seems to be called gcc4-gfortran. Incidentally, you ought to be able to search for this with yum, and that may be the best answer to your question:
yum search "*fortran*"