I have a sore lower back from sitting at my desk too much. What are some good stretches I can do to relieve the pain?

Treat Your Own Back. Much of the cause of lower back pain is from sitting for hours at desks or in a car. Poor posture adds fuel to the fire. Most of the time, our backs are in flexion (bent forward) and too little time in extension (arched). Robin mackenzie, a physical therapist's "treat your own back" has good explanations, excellent exercises, clear diagrams/pictures. It costs about $15.
See picture here. Minor musculoskeletal back pain often get better in 2-3 with simple measures such as some antiinflammatory meds such as motrin/aleve, icy-hot rubs, gentle range of motion and exercises as pictures here can help. If you have leg weakness, numbness, or trouble with bowel/bladder control, or fever, consult doc asap. Good luck.