Starting My own product

Hi all i am a regular reader of Hacker News.I am working for a software firm since 3 years.My long term goal is to start my own company. Now i like to start a product part time to learn how to build a product from scratch, learn new technologies and not for money.I am an average programmer but ready to learn new things. I would like to build up something which is cost effective and as a service something on Google AppEngine.Please suggest me some product so that i can work on .Your kind advice is highly appreciated.

Thanks Kiran