Not really a loophole, but there's a retirement contribution credit for those with lower incomes (less than 59K for MFJ, 29.5K for Single, and 44.25K Head of Household). It's between 10% and 50% of your contribution of up to $2000. So if you are MFJ with an AGI of $35.5K, and each spouse puts $2K into a roth for a total of $4K, they can get a tax credit of $2K. Which can be pretty sweet. Yes, the income limits are low, but if you find yourself in a situation (like mid-year ER), temporary ER, etc, it can be a nice bonus.
Also, I contributed to 401Ks when I was single with a higher tax rate to reduce my taxable income. Then I got married and after a kid we went down to one income. So I rolled all those old 401Ks into my Roth IRA, and paid income taxes while in a lower tax bracket. I can't remember how much I rolled over, maybe 40K? If so, my savings was about 4K. (highest tax bracket was 15% when married with one income, and 25% while single.)