Loosen a stuck knob on a water valve with penetrating oil and heat rather than calling a plumber. Spray oil on the threads, dry with a towel, then point a blow-dryer at the knob to expand the metal and release the knob.
Cost: $5 for a can of Liquid Wrench.
Savings: About $100, based on an average plumbing service call.
Bonus: This trick can also unstick frozen nuts and bolts.