If there's a bunch of people, or if you're coming in late, leaving early, etc. other options are:
Rental Cars:
Cheapest rental car rates I've found for DIA are at Hotwire. They don't tell you which rental outfit it is up front, but they only partner with Avis, Hertz and Budget, so you know it's one of the three. You don't get any airline miles or points, but their rates are typically significantly lower than what I've been able to get directly (unless of course you've got stellar corporate discount rates) or via Orbitz, etc.
Shuttle Van to/from Denver Airport:
Colorado Mountain Express provides door-to-door shuttle service between Denver Airport and Summit County (they can drop you off / pick you up at my place, or can drive you directly to/from the resorts depending on time of day). Costs about ~$60 each way if you book online. Check to make sure their schedule works with your flight times though, as their last shuttle leaves DIA around 9:30PM coming up to Breckenridge, and doesn't get down to DIA until around 8AM on the way out. Also check to make sure there's enough room for you in other cars once you do get up to Summit county ;^)
Other:
Colorado Road Conditions: 303-639-1111