I got this from a friend, who got it from nolo.com
Legal Benefits of Marriage
• Filing joint income tax returns with the IRS and state taxing authorities.
• Creating a "family partnership" under federal tax laws, which allows you to divide business income among family members.
Estate Planning Benefits
• Inheriting a share of your spouse's estate.
• Receiving an exemption from both estate taxes and gift taxes for all property you give or leave to your spouse.
• Creating life estate trusts that are restricted to married couples, including QTIP trusts, QDOT trusts, and marital deduction trusts.
• Obtaining priority if a conservator needs to be appointed for your spouse -- that is, someone to make financial and/or medical decisions on your spouse’s behalf.
• Receiving Social Security, Medicare, and disability benefits for spouses.
• Receiving veterans' and military benefits for spouses, such as those for education, medical care, or special loans.
• Receiving public assistance benefits.
• Obtaining insurance benefits through a spouse's employer.
• Taking family leave to care for your spouse during an illness.
• Receiving wages, workers' compensation, and retirement plan benefits for a deceased spouse.
• Taking bereavement leave if your spouse or one of your spouse’s close relatives dies.
• Visiting your spouse in a hospital intensive care unit or during restricted visiting hours in other parts of a medical facility.
• Making medical decisions for your spouse if he or she becomes incapacitated and unable to express wishes for treatment.
• Consenting to after-death examinations and procedures.
• Making burial or other final arrangements.
• Filing for stepparent or joint adoption.
• Applying for joint foster care rights.
• Receiving equitable division of property if you divorce.
• Receiving spousal or child support, child custody, and visitation if you divorce.
• Living in neighborhoods zoned for "families only."
• Automatically renewing leases signed by your spouse.
• Receiving family rates for health, homeowners', auto, and other types of insurance.
• Receiving tuition discounts and permission to use school facilities.
• Other consumer discounts and incentives offered only to married couples or families.
Other Legal Benefits and Protections
• Suing a third person for wrongful death of your spouse and loss of consortium (loss of intimacy).
• Suing a third person for offenses that interfere with the success of your marriage, such as alienation of affection and criminal conversation (these laws are available in only a few states).
• Claiming the marital communications privilege, which means a court can’t force you to disclose the contents of confidential communications between you and your spouse during your marriage.
• Receiving crime victims' recovery benefits if your spouse is the victim of a crime.
• Obtaining immigration and residency benefits for noncitizen spouse.
• Visiting rights in jails and other places where visitors are restricted to immediate family.
Note that if you are in a same-sex marriage in Massachusetts or a domestic partnership or civil union in any of the states that offer those relationship options, many of the benefits of marriage won't apply to you, because the federal government does not recognize these same-sex relationships.
Got all of the above from nolo.com on an article on family law.