This is from the apple developer discussion boards, not sure if it is any use...
There is only one* free option, g77, the fortran front end to the gnu compiler (GCC). You will need two things:
1. GCC, which is availale from Apple. It is on the Developer Tools disk that ships w/ OS X. If the c compiler works (called cc in Apple's distribution), then you hve GCC loaded. If you did not get a develper disk (ie w/ a new computer), you can download the develper disk from Apple, but that is a bit of a hassle. First you need to join the Apple Developer's Connection at
https://connect.apple.com/cgi-bin/WebObjects/MemberSite.woa/96/wo/Ub5b87W8Ac4W3lL5KSS1M056dYB/0.0
then follow links for downloads and OS X to download the whole developer's disk -- including mostly stuff of no particular interest.
2. Get the g77 front end. There are several sources, but this may be a good time to install fink, which is a package-management tool to maintain various UNIX packages distributed by the fink project; see fink.sourceforge.net. Then, you just enter "sudo apt-get install g77". Once it installs, "man g77" will work and you can take it from there.
The site
http://floyd.liunet.edu/~gkhanna/hpc.html distributes the whole GCC, including g77, but I think you will still want Apple's Developer Tools. If you want to skip #1 above, try this first and see if you get enough of the pieces you need. If you use this distribution, you probably should stick to stable version 2.95.
It would be nice if Apple were to make this all a bit easier in the future; I take it the next releas of GCC by Apple includes the fortran front end.
*(I omit f2c, which translates fortran into c, so is not at all a fortran compiler and does not appear to produce efficient code.)