I am in the process of taking an Introduction to Psychology online through the University of North Dakota. I can't remember for sure but I think you can enroll at any time. You can do the work on your own time, but you need to have 5 exams proctored. Fortunately the reference librarians at my public library agreed to do this for free in the evenings. There is a quiz for each chapter, and the quiz is open book. But in my opinion to do well on the tests you basically have to know everything in each chapter since the test questions are drawn from a big pool of multiple choice questions, some of which are easy but some of which might refer to just one picayune sentence in the reading.
Anyone know a course that allows you to start and end on your own schedule? I did some pre req's through University of Rochester that were like that - super convenient. They don't have general psych though :/