The subject says it all... got about 30 minutes to burn? Check this site out, download the client and fire it off. I've gone through 3 guided tours (see toolbar at the top of the app) and I'm hooked!
DISCLAIMER: this is a HORRIBLE link to hit if you have a ton of work on your plate and are into astronomy. Thank goodness I didn't look at it last night when I was on deadline and up until 3a shipping code!