AFRs are sets of interested rates published by the IRS under 26 USC 1274(d) for several tax-related purposes. The IRS publishes AFRs in a revenue ruling released on or around the 20th of every month. The current ruling and other recent rulings are posted on the IRS Web site (go to www.irs.gov and search for Applicable Federal Rates in the "Search IRS Site for:" box).
AFRs are promptly re-published in Tax Notes Today, BNA's Daily Tax Report and many other tax publications.
Old AFRs: Historical AFRs are available free on the IRS web site back to 2000 (see above) and on the Pillsbury Madison & Sutro web site back to 2000. You can also find historical AFR rates in Checkpoint back to 1989, in the CCH Standard Federal Tax Reporter and by searching an electronic edition of Tax Notes Today or the Daily Tax Report.