No one is happier to see you married than your mother. But your insurance agent is pretty giddy, too.
Car insurers in particular reward those who tie the knot with lower premiums. Further cost savings come to couples who combine auto policies (as well as any other kinds, such as renters', homeowners', etc.), which should get you a discount since you will become a bigger (and better) customer to them.
Where else can officially hitched twosomes cut costs?
Your boss: Closely compare benefit plans for duplication. For example, if your spouse can cover your health insurance, maybe you can opt for some of other options such as a cafeteria plan, additional vacation time, supplemental life insurance, or dental coverage for the dog.
Your bank: No matter what banking setup you two kids choose (a single combined account; two separate accounts; his, hers, and ours...) be sure that your bank knows you're legally bound. Accounts that are linked may qualify for lower fees or higher interest rates -- which usually require a minimum deposit across accounts.
Your bills: Sharing dish-washing duty is only part of living under a single roof. Now you get to split the cable bill, the newspaper subscription, and lawn care charges. What about food and clothes? Take advantage of being a bigger consumer by buying the half-gallon rather than the quart of milk, the case of wine, or the really big bag of Doritos. Costco was tailor-made for couples.
What to do with all that dough you two kids are saving? Find a safe place to put your money for the time being (here's how), and treat your mother to dinner out once in a while.