For every dependent you have, you are allowed to claim a dependency exemption, which decreases the amount of income on which you are taxed. For 2011 taxes, this exemption amount is $3,700.
You may also be eligible to claim the Child Tax Credit, reducing your income tax burden outright by $1,000 per qualifying child.
A dependent will make it easier to claim the Earned Income Credit and also increase the amount of your credit.
If you paid someone else to care for your qualifying child or dependent so you could work, you may be able to claim a credit for child and dependent care expenses.
A dependent may help you qualify for head of household filing status, which features several tax benefits including a larger standard deduction and lower tax rates.