Investment Interest Expense
Interest on debt incurred to carry investments is considered investment interest expense, and can be deducted to the extent of investment income (dividends, taxable interest, capital gains).
Margin interest qualifies. Less obvious is credit card interest, when credit card debt is used to carry investments. Allocation can be appropriate (for instance, if you took an $8000 cash advance to buy stock and another $2000 to go Christmas shopping, 80% of the interest would be investment interest). Transaction fees (on cash advances or balance transfers) are an investment expense (miscellaneous itemized deduction subject to 2% adjusted gross income limitation).