Lots of little cities and bigger towns surrounding Boulder and Denver are great. Boulder and Denver have lots of bike lanes, an established bike community too, but the automobile traffic is a bit too much for me to ride in every day. Some other people mentioned Golden and Colorado Springs. Those towns are great for bikes. Lots of pro and Olympic type training programs are based in Colorado Springs.
My favorite biking area area in Colorado is Fort Collins and Loveland, and the little towns that have become suburbs around them. It is a lot like Boulder as far as having a large biking community, lots of bike lanes, and lots of great prairie and foothill/mountain routes from your driveway; but with a lot less traffic, and the automobile drivers are much less aggressive in general.
There are definitely better places to visit and bicycle on vacation (parts of France, Italy, Spain, California, etc.), but this area is also relatively affordable to live in.