What good PowerPoint editing apps are there for iPad?
Try:


Keynote (not 100% compatible with powerpoint; some things are lost in the translation from Microsoft's to Apple's format)
Documents To Go
Office2 HD
QuickOffice


Personally, I prefer Keynote over what I've seen of some of the others, but they all have their limitations. Keynote's just the one I have the most familiarity with.