I've noticed a trend of late where more developers are building and open-sourcing not just controls but entire apps. I think this is fantastic, and was delighted to see Ink build and open-source a collection of apps this past week.
ThatInbox "is simple, elegant, and free new way to do your email on an iPad." - This app shows off how to use the MailCore library to build a full-fledged mail client.
ThatPDF shows you how to view, sign and annotate PDFs from an iOS app.
ThatPhoto shows you how to work with photos on iOS, including editing them with Aviary.