Meritline offers the Soundlogic 2,600mAH Portable Backup Battery in Blue, model no. BB26-24/5722, for $19.99. Coupon code "MLCY11UN" cuts it to $14.99. (GearXS via eBay charges the same.) With free shipping, that's tied with our mention from a month ago and the lowest total price we could find by $5. It charges and provides backup power to USB input-compatible devices, including iPhone, iPod, and most other current smartphones.
Update: Our mention at Meritline has since expired; however, GearXS via eBay still offers it for $14.99 with free shipping.