Potts Track, named after Frank Potts, a CU track coach from 1927 to 1968 (not to mention CU's football coach in 1940 and 1944-45), is located just east of 33rd Street, between Colorado Avenue and the Boulder Creek Path (you can reach it from Boulder Creek by means of a dirt path or from Colorado by means of Innovation Drive). As of 2007, CU's track is open to the public from 6:30am to 9:30am Tuesdays and Fridays and from 5:00pm to 6:30pm on Wednesdays, from February through late October (the track is often left open after those stated times, though I wouldn't plan a key workout around that fact).
Potts Track is the center of the Boulder running world, which of course is the center of the world of running (stick it in your ear, Eugene, Oregon). Show up at the right time, and you'll rub shoulders with the best runners in the world, up to and including Olympic marathoners from the United States and other nations.
NO SPIKED SHOES ALLOWED!