I'm going to disagree with dagamer on this one. I have an iPad and for father's day, my family bought me a stylus. I will agree that the lack of a true pressure-sensative digitizer results in less than optimal sketching capabilities, but for just taking notes, a stylus and an iPad work well enough.
Notability is my favorite.
http://www.gingerlabs.com/cont/notability.php $1 - here's a video of it.
http://www.youtube.com/watch?v=wV7-3MGIF0s
Penultimate is very good and free.
https://itunes.apple.com/US/app/id354098826?mt=8&ign-mpt=uo=4
Paper is more for sketching than writing, but it's free and well done.
https://itunes.apple.com/app/paper-by-fiftythree/id506003812
The only one that I currently have loaded and use regularly is Notability.