As other posts suggest, it makes a difference whether you’re salaried or self-employed. If you’re self-employed and filing a Schedule C, most business expenses, including office equipment, are fully deductible. If you’re an employee, it’s not so simple. You can only claim that keyboard if you itemize deductions, and you can only deduct the portion of your work-related expenses that exceeds 2% of your AGI (adjusted gross income). So if your AGI is, say, $50,000, you can’t deduct your first $1,000 of employee expenses.
Basically, this means that if you’re self-employed, which could be a business on the side in addition to your day job, you’re good to go. If you’re a working stiff, you’re probably out of luck.