Depending on your particular situation, you may be a candidate for an Offer in Compromise or one of two payment plan options—an installment agreement or currently not collectible status.
An Offer in Compromise is a somewhat complicated process that allows you to offer less than the total amount owed based on your complete financial and social picture in return for maintaining tax compliance for the following 5 years. It is final—if awarded, your tax debt is permanently reduced.
An installment agreement is available to most people and may be made monthly over up to 5 years. This reduces the payment burden but not the tax debt itself. A Partial Pay Installment Agreement is based on your financial ability to pay over six years and if approved will reduce the the total liability if your financial situation doesn't change during the installment agreement. Any tax refunds during the installment agreement will go toward the balance due.
Currently not collectible status is a form temporary relief granted by the IRS to those whose financial situation does not allow them to make payments. To be awarded this relief, specific paperwork is required to be filled out and income and expense information provided to the IRS. This does not reduce the tax debt unless the change in financial situation is permanent—currently not collectible status may be renewed indefinitely although proof of income/expenses may be required anytime at the IRS's request.