On of my favorite websites for electronics circuits is electronicsforu.com. You will find very good documented circuits that include a detailed explanation of how they work. I think this archive can provide a great library of circuits to devise your own ideas from. Or have fun building one as a project on the weekend. Some are ardunio based, and others could be converted by you for use with your ardunio.
If you enjoy building robots, here is a good writeup on the hacker lifestyle at http://www.catb.org/~esr/faqs/hacker-howto.html by Eric Steven Raymond.