Walmart offers the Stanley FatMax 140-watt Power Inverter, model no. PCI140, bundled with a Freestanding Keychain LED Light for $19.98. Opt for in-store pickup to dodge the $5.97 shipping charge. That's the best deal today by $11 and tied with last month's mention as the lowest price we've seen. It plugs into a car cigarette lighter and features a 1-amp USB port, two AC outlets, 3-foot cable, and mounting kit.
Of note, Amazon offers Prime members a similar Stanley FatMax 140-watt Power Inverter (no bonus keychain) for $19.98 with free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, shipping costs $6.78.) It's $3 off and also the best price we could find. Its features are similar to those above except it has two USB ports and one AC outlet. Update: It's expected to ship in one to two months.