The first one is the government's site and I'm also including their 'contact us' page. It gives the address you can write to them as well as an email address. All of the phone numbers are automated. Here you can read all about the change to digital and the converter box coupons that you can order.
There's a terrific informational site called TV Fool where you can input your zip code and find out what channels you are most likely to receive once the transition is made. It divides the channels into ones you should be getting, one you will probably get, and ones you probably won't without a really strong antenna. The TV Signal Locator page is extremely simple to use and only takes a minute.
Next up is Rabbit Ears and it can tell you what your local stations are and where their towers are located. It's more neat information.
Last up, if you have questions about the conversion, there's a forum designed to answer your questions. It's not government run, but there are some very in-the-know folks there and it's another good place to ask questions about the entire process of switching to digital and setting up your converter box.