Math Grad Handbook - This was created by graduate students for graduate students. There's great information here about all sorts of things.
Center for Teaching Development (CTD) - An interdisciplinary group whose goal is to improve teaching on campus: they hold seminars and workshops, observe and consult with TAs and professors, and make available a large library of multimedia resources related to teaching.
UAW Local 2865 - The union to which the TAs belong, as well as some 10,000 other academic student employees. There's information on bargaining rights, grievances, and other union-ish type stuff.
Office of Graduate Studies (OGS) - They deal with all aspects of grad student life, including TAing. They have useful resources on grading and dealing with academic dishonesty, fee remissions, etc.
Office for Students with Disabilities (OSD) - Information designed to promote effective student planning and appropriate academic accommodation for students with disabilities.
Counseling and Psychological Services (CAPS) - If a student seems to be having personal problems that are affecting his or her schoolwork, or if you are concerned for a student's mental well-being for some reason, these folks probably know what to do.
Office for the Prevention of Harassment and Discrimination (OPHD) - Primarily provides assistance in investigating and resolving complaints of harassment and discrimination. They have good resources on prevention and early resolution, which may be useful for TAs.
Math People - The UCSD math department listing.
Academic Integrity Office - Forms and policies regarding how to handle suspected cheaters.
Associate Instructor Guidlines - This is a document prepared for students working as Associate Instructors.