Portable external battery pack for Macbook Air

The Haswell Macbook Air has amazing battery life, but there are times where an electrical socket may be too few and far in between, literally. I'm wondering if any of you travelers have used a product for this purpose, and if so, what do you recommend.

I think there was a company that made a variety of sizes of external batteries for Macs a while back, but I think Apple sued them or something. Forgot their name.