You have alot of options even in the $500 price range. Here are a few I can think of.

1. If your hard rock is too big you could try and put up a Craigslist listing wanting to trade for something a size smaller. In my experience there are always a shortage of bigger frames so this might work out pretty quickly. If you decide you want something more road oriented then you will have a fun bike to play around on trails too.
2. Vintage road bikes can be awesome commuters and you can get nice ones. How much do you know about bikes and are you willing to do some work yourself? Buying used can be great, btu if you don't know much the savings of buying used can get eaten up quickly with a bad purchase.
3. Look for a new bike from a online source. Bikesdirect.com is a great place to look. http://www.bikesdirect.com/products/road_bikes.htm
4. Look for a used modern bike including hybrids. Once again a word of caution here, but there are deals.
5. Buy a hybrid at a bike shop. You could get one in your price range. Sometimes you can get lightly almost un-used hybrids on craiglist for fantastic prices.

Anyway, its great you are joining the ranks fo commuters and you will get lots of great info here.