Here are some resources that I've found helpful at various points during my career.
Arithmetic geometry,
number theory, and
algebraic geometry.
Computer algebra software: Magma, Sage, Macaulay 2.
C.V. templates: mine and Ezra Miller's.
Website templates: Open Web Design and Start Bootstrap.
Text editors: TextMate and Sublime.
Ten Lessons I Wish I Had Been
Taught by Gian-Carlo Rota
Tips of giving talks by Jordan Ellenberg
How to Give a Good Colloquium
by John E. McCarthy
Practical suggestions for mathematical writing by Bjorn Poonen
Advice for the Young Scientist by John Baez
Tips for asking for recommendation letters by Ravi Vakil
General advice:
UC Davis,
Bjorn Poonen,
Ravi Vakil, and
Terence Tao
MGSA's list of past qualifying exam questions.
My qualifying exam syllabus.
Past Princeton General Exams.