/ jansonius / scientific software / compilers
Free fortran compilers for DOS are scarce. BC-fortran is a small fortran compiler written by Andre Koestli. It should run on any IBM compatible PC including an XT. G77 is a fortran compiler from the Free Software Foundation (GNU). When I found this DOS port, it was combined with the EMX DOS-extender of Eberhard Matthes. As EMX doesn't function without EMM386, I replaced EMX by CWSDPMI/RSX.
A famous Pascal compiler for DOS, Turbo Pascal 5.5, can be found at the Borland Museum. Unfortunately, when using TP5 on a PC with a "fast" processor (typically above 200 MHz), the well known runtime error 200 occurs. For more information look here. A bug fix can be found here.
I don't like C. However, sometimes you may have to compile a code written by someone else. C compilers can be found at the Borland Museum and here.
Several programs as offered on this site were written in QuickBasic 4.5 (the compiler QuickBasic should not be mixed up with the interpreter QBasic as included in MSDOS). You need only three files to run QuickBasic: bc.exe, bcom45.lib, and link.exe.