But I owe the Fed and can't pay now!
You are required to pay your tax liabilities by April 15th but, you can pay through an IRS installment agreement although penalties and interest will be added. (Click for the IRS installment agreement form 9465)
You can also pay your taxes by credit card via the two (2) authorized web sites. It'll cost you a fee (about 3%) to use your plastic, but it's cheaper than paying IRS interest and penalties!
Lastly, plan ahead for next years taxes by reviewing what caused the problem in prior years and adjust your withholding or adjust your taxable income or seek assistance with your situation before tax time. (that's a hint...)