It's no secret that I use an Apple PowerBook G4 (15-inch, 1.67GHz if you must know) along with my IBM/Lenovo ThinkPad T40.  I used to be able to get over 2 1/2 hours of use out of the G4's battery before I had to plug it in, but now it conks out after 1 1/2 to 1:45.  Still not too bad, but starting to get annoying.  Battery life goes way down while I'm using a WWAN card like the Verizon PC 5220. 

I was sent a Newer Technology AL15 55.5WHr battery the other day (the standard battery was 44WHr) from MacSales, and it is like having a new laptop.  I can now surf for about 3 hours, though watching a DVD movie will top out at about 2:30 (long enough to get through most movies, aside from Titanic or one of the Lord of the Rings movies).  The battery even has the Apple battery's best feature, the 5-LED power indicator on the bottom, that's accessible even when the PowerBook is on.  The AL15 55.5WHr is $139.99, not bad considering the Apple replacement battery is $129.99 and has less capacity.  It's not quite the 5+ hours I get on my ThinkPad with extended battery, but not bad just the same.