Go to http://www.ogame.org . It's free, and all you need is a web browser to play.

Basically, you develop your planet's resources over time and eventually get to where you can invade other players' planets. I'm still developing my infrastructure after about 3 weeks of playing and haven't had any interaction with other players up to this point. I've played fancier MMOGs in my time, but this is great for its simplicity, and you can easily play it at work without drawing any attention. Plus, it's free.

I'm Bruno on Universe 1. My planet is Samsara, located at [1:93:9]. Maybe in about a year or so, I'll be kicking ass all over the universe and have my own Galactic Empire.