There is at least one vineyard near Denver, though i don't think it's in the city limits. It's about 4 acres planted with, i think, Cab Sav and Merlot, and is located at about 64th and Washington. The area is light industrial, but the vineyard and winery owners, the Balistreri's, have lived there for probably a century now. The area, then known as the Welby district, was agricultural and known for its production of sweet celery. (I don't know exactly what the celery was called, but one Balistreri told me that it was white and sweet and in high demand at Thanksgiving in those days.)
You can visit the Balistreri winery. They produce several wines, many of them very good. (Disclaimer: I know the owners and like them very much, so I'm a bit biased.) They also throw a great wine tasting party in October, complete with whole roast pigs and a wine stomping for children. The wine is usually bottled and called "Little Feet" wine.
For information about Colorado wine, go to
Colorado Wine Industry Development Board. You can click on the Wine Trails link to find PDFs of Front Range winery locations. There are several wineries in Denver; most grapes come from the Palisade area, as denverian pointed out, though there are two other AVAs in CO. There are also three wineries in Boulder. (Another disclaimer: I work part-time for one of them.)
One last suggestion: Always call ahead to a CO winery you want to visit. They are all small operations and so may not always have staff to help you with your wine tasting.
If you do try our wineries, enjoy! I think you'll be surprised at the quality--but then I'm biased.
Oh, I should have said also that the wineries are not at all like Napa. The vineyards, for the most part, are elsewhere. There are only the wineries where the wine is made, but i think that was pretty obvious from the info above, huh? And glitzy winetasting rooms, like you find in Napa, don't exist on the Front Range. The only glitzy one i know of is Two Rivers in Palisades. And, if you want to have something to eat at one of the Front Range wineries, I suggest you go to Creekside Cellars in Evergreen. The winery has a deck overlooking Bear Creek and a great selection of light meals. A great place to while away a summer day. (Yet another disclaimer: I know the owners of Creekside, as well. The CO wine industry is rather a small group of individuals. You sort of get to know each other, at least by sight, from attending the same meetings and events.)