Today, Woot.com has the Energizer Energi to Go Portable Mini and Micro USB Charger (2 pack) on sale for just $3.99. Shipping is $5, so that makes each charger only $4.49!
This is a great deal, as these products retail for $10+ each elsewhere! Be sure to read the specs on the bottom of the page to verify they will work with your device(s).