Roommate situation: we each paid 50% of the rent. Roommate skipped out on the last 2 months of rent, which made me accountable for the full amount. Is this a write off?

Unfortunately, the payment of rent on your personal residence is not a deduction on your federal income tax return.  However, some states give a renters credit.  Check with your state to see if it is offered.  

There is not a rent deduction or credit on your Federal return.  If your state has anything for renters you will be prompted to enter your rent info when you complete your state return.  As far as I know, the states that have anything for rent are Arizona, California, Connecticut, Hawaii, Indiana, Iowa, Maine, Maryland, Massachusetts, Michigan, Minnesota, Missouri, New Jersey, Rhode Island, Vermont, and Wisconsin.