A nonqualified deferred compensation plan offered by a state or local government entity, or by a nonprofit/tax-exempt organization that is not religious, e.g. a hospital or university. These plans are controlled by Section 457 of the Internal Revenue Code, and are somewhat similar to nonqualified deferred compensation plans that for-profit companies can offer under Section 409A. One big difference is that Section 457(f) taxes compensation when it is no longer subject to a substantial risk of forfeiture, i.e. it does not defer taxes until distribution, as with for-profit plans under 409A.
For the details of Section 457 plans, see the related article elsewhere on this website. Nonprofits can also offer Section 403(b) plans, the nonprofit equivalent of the 401(k) plan at for-profit companies.