Insider's Guide to Graduate Programs in Clinical and Counseling Psychology
This book and the prior two resources are geared towards clinical psychology programs, but they also contain a lot of good general information, including descriptions of different types of psychology graduate programs, different parts of an application, and an application timeline.
Business PhD Wiki (e.g., for organizational behavior, the business school version of I/O psychology)
Summer research opportunities
These two are just an example of the many opportunities that are out there! You can also reach out to professors - either at your school or other nearby universities (or even some of the professors you might be interested in working with!) - and see what opportunities they might have open.
If you're an incoming or first/second year graduate student, you should considering applying for the National Science Foundation (NSF) Graduate Research Fellowships Program (GRFP). The NSF GRFP provides funding for three years, as well as great research and professional development opportunities, which is pretty awesome. The application is typically due towards the end of October each year. Here is more information on the program, as well as resources that I found helpful when applying:
The Ford Fellowship is also a great opportunity for students who are part of groups who have be historically underrepresented in academia (i.e., Alaska Natives, Black/African Americans, Mexican Americans/Chicanas/Chicanos, Native Americans and Pacific Islanders, Puerto Ricans). It also provides funding for three years. See more here.