Investment portfolio or savings – Income is generated from one’s investment portfolio, which is solely dependent on the stock and bond markets, such as a 401(k) plan, IRA or other accounts which may include mutual funds, individual securities, etc.
Employer benefits – Some still have an employer benefit (such as the Federal or State government, a large corporation, etc.) such as a Defined Benefit retirement plan which will send monthly checks after you retire.
Annuities – Another effective strategy can be variable annuities that offer lifetime benefits without annuitizing (via a Guaranteed Income Rider), which ultimately pays the policy annuitant a monthly income benefit for life, and the beneficiary receives the balance at death.
Rental and/or income property – Working assets that generate a constant flow of income, such as real estate, a trust or other account or inheritance property.
Notes Receivable – Can include private notes that others owe you or even arrangements with another party that can provide a set monthly income.
Timber proceeds – In the case of land ownership, periodically a timber cut or thinning can provided additional cash flow.
Oil/Gas partnerships – Can provide periodic income to the limited partner who assumes no liability beyond the funds they contribute to purchase units in the partnership.
During retirement the best of both worlds is at least stable income, but with some potential to increase over time. It is quite possible diversifying your retirement income may help produce that outcome.