This page is largely out of date and reflects the optoions in the PowerPC days. For current information, go here:
1) intel -- best perforance, not cheap
2) gfortran -- free, part of the GCC compiler suite
3) g95 -- free, splinter project from GCC
I currently use gfortran on my laptop. You can get gfortran and g95 from their web pages, MacPorts, or Fink.
Apple has a page on OS X compilers and other development tools. Apple's tools, however, are directed more toward software development than conventional scientific programming.
GCC 2.95 is included in the Apple Developer Tools, but without the g77 frontend. Developer Tools comes with all the high-end systems. It is freely downloadable from their web site if you did not get it with your computer (eg iMac iBook). On my powerbook, the Tools package was on the hard drive, but not installed.
The g77 frontend for GCC 2.95 is available from several sources. Easiest if via fink. Otherwise, see link in next item.
You can get the latest GCC 3.1, including g77 and lots of other high-performance computing stuff, from Gaurav Khanna's page on High Performance Computing but you'll still want the Apple Developer Tools.
The only modern (ie fortran 95) compilers available for the Mac are from three commercial vendors.