Language matters even in scientific writing. This is a useful resource if you think you may be making a usage error, or if you want an independent perspective on things I point out when I comment on your writing.

This is a tremendous resource on mathematics. It almost takes the place of the trusty old CRC Math tables that I had to carry around as an undergraduate. It includes entries on many topics of practical use in physics, including trigonometry, including trigonometric identities, and on derivatives and integrals, including tables of elementary forms.