I really like Instapaper. Flipboard is very pretty, although I don't use it. Angry Birds is great, although I didn't install it on my new phone b/c it sucked up too much time. I love an iPad app called Dragon Box, which helps teach kids algebra.
Instagram is neat, but I actually don't really like the interface. But everyone else seems to love it, so maybe I'm crazy.
Fruit Ninja is awesome, but I keep burning my finger so I stopped playing!
What are your favorite iPhone and iPad apps?