AnkerDirect via Amazon offers the Anker 21-watt Dual USB Solar Charger for $50.99 with free shipping. That's the lowest price we could find by at least $9. It's compatible with most USB-powered devices and features a weather-resistant design.
Update: As a reader found, Anker Direct via eBay has it for $50.99. Add it to your cart and it falls to $40.99. With free shipping, that's $10 below the above mention and the best we've seen.