You can now call listed US numbers for free by dialling +1-800-FREE-411 (+1-800-373-3411) via SkypeOut and following the voice prompts. A few caveats: the number you're calling must be listed, and you must provide the name, city, and state of the party you're calling.
In case decades of telco propaganda actually bother your conscience, fear not: Free411 is ad-supported. Perhaps someday all traditional telephony will be. That is, before VoIP kills it.