JuiceDefender - battery saver
It's not the LAST app I used yesterday, but I'm out of sections to describe my Day in App Life! This one is important in keeping me going through more or less constant phone usage. I run this one all the time to keep my battery in shape. When I'm playing with the phone all day, this one's a lifesaver for conserving battery, closing out background apps I'm not using and forgot to close, and setting power profiles.