Vasily's IDL Page - A great compilation of everything IDL, from Vasily Belokurov (IoA, Cambridge). Includes links to common IDL libraries and tutorials (see the top of the page to get started).
gfortran - Links to builds for the gfortran compiler for different operating systems. For Mac, I recommend using the "nice Apple-style installers" included in the MacOS section.
Street Fighting Mathematics [pdf 1.1 MB] by Sanjoy Mahajan
A guide to order of magnitude techniques and mathematical estimation. This is a complete pdf of an early version of the book.
Caltech Ph101 Caltech's order of magnitude physics course. Problem sets have good examples