I have the Ravpower 10000mah and works great for my travels. I noticed on slickdeals that they're having a timed promotion right now for $30. It's from an odd time between 5-8PM PST. But this seems to be a really good deal if you're able to grab one.
(copy and paste, don't click or it will direct you to a broken link)
I'm also considering getting a solar one like the Scosche SolBAT II Solar Powered Backup Battery
. Something like this would be great for being outdoors.