There are many options for yoga in Boulder; it seems like everyone does yoga here. You can take free classes at Prana on Pearl Street or at Lululemon at the 29th Street Mall. The Colorado Athletic club has several classes a day for members. I really like the Yoga Pod at the 29th Street Mall. Matt Kapinus is one of their instructors and is excellent as are most of their instructors. If you are looking for a yoga studio to join, most will allow you to try them out for free for a few classes until you decide. Namaste.
Good question and hard to answer--there are dozens and dozens in Boulder alone. My best friend swears by Core Power Yoga. I go up to Lyons (north 20 minutes from Boulder) to get away from the city to Mayama Studios-a sweet quiet studio.