Any bicycle can be used for commuting, and your own choice will depend a great deal on your riding style and needs.

Sizing is very important. your inseam / stand over is the first dimension to consider, and your torso length / arm length will be factors as well.

Generally speaking, though, features that are generally considered beneficial for commuting by bicycle (exception, racing-oriented riders who will desire more performance)
- durability rather than lightness
- The ability to install fenders and racks
- good brakes (any type will do, but quality is beneficial)
- puncture resistant tires that offer good control and comfort rain or shine. Most non-racing types will opt for tires 28-38mm wide with slick or semi-slick tread.
There are so many variables.
Drop bar or flat bar? personal choice for sure. Drop bars offer more hand positions, flat bars tend to promote a more stable riding "feel" in more conditions.

Anyway, test ride a variety of bicycles at your local bike shop to get a sense of what style you feel most at home on. Let them know you are looking for a reasonably priced bike for commuting.

A quick peruse through craigslist in your area:
http://raleigh.craigslist.org/bik/1848564719.html sporty road bike, narrow (25mm) tires - more of a racing bike than a commuter, but a solid value for the price. If the frame would allow 28mm tires, you could throw on some "roadie" type fenders that mount to the stays, and use a backpack or seat post rack. This type of bike would likely feel "nervous" and not as comfortable as, say, a hybrid or mountain bike.

Ads like this http://raleigh.craigslist.org/bik/1848439774.html can be a dead end or a "barn find" (great bike that has been in someon'es garage for 30 years). Unless you have a friend who knows what to look for, I'd avoid ads like these.

Heres a very strong candidate. http://raleigh.craigslist.org/bik/1848171060.html You'd possibly want another saddle, and would probably want to give it a good once over (maintenance inspection at a bike shop) to assess what needs to be replaced. Common candidates: brake pads, chain, rear cassette. All manageable. If the sidewalls of the tires are dry/cracked, you'll want new tires. All in all, this is a great possibility.

Singlespeed mountain bike that could do double duty (short commute plus fun bike on the trails) http://raleigh.craigslist.org/bik/1847797114.html The gentler gear ratio would probably be manageable on your long, low grade hills.

crappy photo, but if this is really from Performance Bike Shop (kind of like Bike Nashbar), it could be a good deal. http://raleigh.craigslist.org/bik/1847201588.html

Slick tires, rack and fenders, perhaps a few maintenance items, this one would be another excellent candidate: http://raleigh.craigslist.org/bik/1846808701.html

Decent old school mountain bike, WAYYY overpriced IMO - it would be a good value at $100-$125. http://raleigh.craigslist.org/bik/1846192918.html

BINGO! Another VERY solid candidate http://raleigh.craigslist.org/bik/1846170325.html

Big frame, but one of the better grade old school mountain bikes http://raleigh.craigslist.org/bik/1845032778.html Good possibility it you are tall.

On the other end of the size spectrum, in a solid older mountain bike: http://raleigh.craigslist.org/bik/1841191145.html

$350 seems steep, but this is a possibility: http://raleigh.craigslist.org/bik/1841080761.html

Solid bike. No eyelets for rack or fenders - fairly easy to work around. http://raleigh.craigslist.org/bik/1839738423.html

Anyway, there's a bit of a starting point......

Brands to avoid at all costs:
Next, Murray, Huffy, Roadmaster