Portable Mac Charger?

[doublewinder]

I frequently am on my Mac away from the desk. However I run out of juice in a few hours or sooner requiring me to unhook and cart the adapter from my office. It's inconvenient to keep plugging and plugging it...and would be a lot easier if I had some portable charger just for when I have the Mac away from the desk.
Do any of you have any products to recommend that aren't that expensive and work well?

[Morkle]

Can't you just use the Mac charge plug? Maybe buy a new battery depending on what Mac you have. You could replace them up until 2013? If I remember correctly?