I recently did a Program Score Card for the Southwest Rapid Rewards program and was pretty impressed. Its a generous program that gives a free flight for every 8 roundtrips. Additionally, when you hit 100 credits in a year (whether by flying or credit card spend), you get a free companion pass that is good for 13 months. That is HUGE if you can take advantage of it.
For a limited time, new Rapid Rewards members can earn 4 free credits just for signing up and opting in to receive program emails (link).
Its a great start to earning a free flight and it gives you 1/25 of a companion pass!