Some workplace retirement plans such as a 401k or 403b do allow you to contribute after tax monies.  This is not very common as money generally goes into these plans as pre-tax contributions.  Now a recent IRS ruling will allow you to rollover those after tax funds into a Roth IRA.   When you leave a job or retire-make sure to ask your plan provider if there are any after tax monies in your account.

Here's Morningstar view on after tax rollovers:

Do not overlook this unique opportunity to get a Roth IRA "tax-free!" The only reason NOT to take advantage of this is if the employee knows he needs the cash immediately for some reason, in which case the new IRS rules allow him to have the aftertax money paid to himself (tax-free of course) separately, while sending the pretax money via direct rollover to a traditional IRA.