The main purpose of my bike will be commuting to work 5 days a week probably 10-20 miles a day. It will need to handle lots of rain, maybe some occasional light snow, and be able to handle all my gear. I also will take it for some leisure rides. These may be some short 10 or 20 mile rides where I'll push my self to see how fast I can go or some longer 50 mile to 100+ mile rides where I'll make a day of it. One day I would like to do some longer rides over a few days.
One option is to get a racing bike and equip it for my commute (rack, fenders). Although one of these would be best for any ride that takes less than half a day, the are not ideal for fenders or riding in adverse conditions.
A cyclocross bike is a road bike that is designed to race in the mud and crappy conditions. They have wider tires and more clearance for fenders. Usually they can convert easily into a good commuter that can handle bad weather. I've seen studded tires for riding in ice and snow that can fit on these bikes.On the downside they are not as quick. I like going fast. That said I'm sure they are still probably quicker that the bike I have now.
A third option is a touring bike. These bike are designed to be weighed down with a lot of pack and be comfortable for long cross county trips. They may be the best option of these three for commuting.
Below are some of the bike I'm thinking about getting. To really be able to make a decision I'll need to go to some bike shops and test the bike out. If it's sold at REI I'll also get about 10% of the price back in my member refund ( I think all but the TREK can be found there). I'm leaning towards a cyclocross bike