Open-Source Examples of well-designed Android Applications?

All of the applications delivered with Android (Calendar, Contacts, Email, etc) are all open-source, but not part of the SDK. The source for those projects is here: https://android.googlesource.com/ (look at /platform/packages/apps). I've referred to those sources several times when I've used an application on my phone and wanted to see how a particular feature was implemented.

In addition to other answers, I recommend you to look at this list:

14 Great Android apps that are also open source

For me, NewsBlur, Hacker News Reader and Astrid were the most helpful. Still, I don't know whether they are "suitable for basic learning".

Are the Android samples not good enough? I've found the ApiDemos to be indispensable when learning a new aspect of Android, myself.