Many of the courses listed on lagunita.stanford.edu/courses run periodically. If you have registered for an account, you'll be notified by email whenever new courses are announced. Just make sure the monthly emails from Stanford Online (at the no-reply address StanfordOnlineemail@example.com) aren't being filtered into your Spam or Trash folders.
You are also welcome to sign up for courses even if their official class end date has passed, provided the course has not chosen to set an enrollment end date. The course materials may stay available, but you won't be able to earn a Statement of Accomplishment for your work.