I can hear the collective groan of thousands of "night" people when I suggest that running in the morning (or even pre-dawn) may be the best time for many people. There are a lot of reasons for this, and many are neatly summarized in this post at the Predawn Runner blog by Greg Strosaker.
Overall, I'm pretty impressed with his rationale for predawn running, particularly how it allows you to engage in a great fitness activity without it interfering with other aspects of your life or the lives of your loves ones. In the summer, it's great that it's the coolest time of day, too. In the winter, however, it's also the coolest time of day, which can mean it's damn cold out there! Still, if you're willing to try a treadmill or perhaps a gym membership, you might have some predawn options year round.
Personally, I'll go predawn in the summer, but midday in the winter. I have yet to feel compelled to buy a treadmill or a gym membership on my budget.