The Franko wallet was one of the easiest wallets to create.  It did get more complicated after block 400,000 when it employed the Kimoto Gravity Well as its difficulty retarget algorithm.

This app is available on Google Play:
https://play.google.com/store/apps/details?id=hashengineering.franko.wallet

Source Code is here:
https://github.com/HashEngineering/frankoj
https://github.com/HashEngineering/franko-wallet

Franko Forum Post(s):
http://forum.frankos.org/index.php/topic,41.0.html

Other Links:
https://bitcointalk.org/index.php?topic=389138.0
http://www.appbrain.com/app/franko-wallet/hashengineering.franko.wallet
http://www.appszoom.com/android_applications/finance/franko-wallet_jizmv.html
http://www.reddit.com/r/Franko