FORTRAN code can be called using MathLink or .NET/Link (see the link for a worked examples).
But as mentioned in a talk by T.Gayley and J.Klein in Wolfram Technology Conference 2011, LibraryLink, which is new in version 8, is much faster. In the presentation, they gave a comparison of these three different approaches. (see also here a comparison by @Szabolcs). I just copy the pros and cons of LibraryLink from that presentation:
Extremely fast, comparable to internal implementation in Mathematica itself.
Memory for large arrays can be shared with the kernel.
Code generation tools in Mathematica can be used to simplify creating and compiling libraries.
Probably a better alternative than installable MathLink programs for most users who want to call C functions.
Need to write specialized C code for every function you want to call.
Without special (and sometimes inconvenient) effort by the author, the kernel is not interruptible while calling a library function. This means that functions cannot be aborted while in progress, and the front end will not be responsive to most buttons, the help system, etc.
The advantages arouse my interest. But I have little experience with C programming... :-( So I want to know how can one call fortran functions and subroutines using Librarylink? Is there any worked example?
Practically, I want to use it to call a cernlib package MINUIT written in FORTRAN 77, see my question here. MINUIT is a complicated package for function minimization and error analysis, and it has been widely used for may years, especially in the fields of nuclear and particle physics. Here is a link to its reference manual.