While open houses do represent a way to see houses in neighborhoods in which you have interest, not all listings are held open (at least not in Denver). Many agents feel that open houses are a waste of the seller's time, as a very low percentage of homes are actually sold off an open house.
IF you find a quality Realtor, and work with that agent to hone your list of house "must haves" needs and wants, the Realtor can do all the work for you; greatly narrow the field to those homes that specifically meet your requirements, and show these to you, even if they are not held open on the weekend.
Hope this is helpful. The sites listed in other posts will give you many of the homes held open, and simply driving neighborhoods (a good idea anyway) to look for signs will be even better.