Some retirement plans allow you contribute to a Roth 401(k), subject to the same contributions limits described in Pre-tax 401k Contribution discussion. Although your contributions are taxed before entering your account; principal, interest, dividends and capital gains are tax free (not deferred) when withdrawn. You may be able to pay lower taxes now in order to avoid what would have been higher taxes when you take withdrawals.
Since the IRS places modest contributions limits (subject to income bands) on Roth IRAs, Roth 401(k) contributions provide a back-door means to accumulate future Roth IRA balances following an IRA rollover. Roth 401(k) balances (including principal and gains) can be rolled over to a Roth IRA, where withdrawals are also tax free. Furthermore, Roth IRAs are not subject to required minimum distributions (RMDs) for the original owner or the surviving spouse. Avoiding RMDs is a valuable income and estate planning strategy.
Next topic - after-tax 401k contribution.