Denver International Airport is a major hub. It is the fifth busiest airport in the US and 13th in the world. It has nonstop flights to 180 worldwide destinations including and 23 international cities (e.g.,London, Frankfurt, and Reykjavík–Keflavík in Europe). It handles an average of 1,600 flights daily. Roundtrip ground transportation on airport shuttles from the airport to downtown Denver is typically about $45-50.
The are many ways to commute from DIA to Fort Collins:
Rental cars at DIA: http://locations.budget.com/us/co/denver/den.html