Jefferson County Open Space Parks (always free)
Jefferson County Open Space Parks are probably the best park value in Colorado. A tremendous variety of parks and activities, such as hiking, mountain biking, climbing, and geocaching, all free to anyone.
Here is a list of their parks, maps, details, etc... 
Snowmass Summer Concerts
On summer Thursdays and Fridays the laid back atmosphere of Snowmass Colorado comes alive with their free Snowmass Village Summer Concerts. The music, set up on a Snowmass ski run stage near the upper village, is loud and wild.
When:
Summer Thursdays (and sometimes Fridays) from 6:15 to 8:30pm.
Where:
Snowmass upper village.
Schedule:
The 2013 Snowmass Village Free Summer Concert schedule is as follows:
June 20: Freddy Jones
June 27: The Dunwells
Friday July 5: CJ Chenier
July 11: The Revivalists
July 18: Mountain Standard Time
July 25: J.Boog
August 1: Tab Benoit
August 8: Nigel Hall
August 15: Robert Walters
More Info from the Aspen Snowmass web site:
Buffalo Bill's Birthday (February)
Golden, Colorado celebrates Buffalo Bills birthday in grand style every year at Rock Rest Lodge. This free festival features re-enactments of the wild west days as well as great music, food, and western authors.
Denver Cinco De Mayo Festival (May 3-4, 2014)
One of the best ways to experience Denver's rich Hispanic heritage is to visit the annual free Denver Cinco De Mayo Festival, one of the best things to do in Denver on a May weekend.
Old Globeville Days and the Orthodox Food Festival (annually in July)
The Old Globeville Days Orthodox Food Festival celebrates Denver's international immigrant heritage with a diverse selection of free entertainment and international food. The music features outstanding performers of diverse folk heritage including an amazing Celtic performance. The incredible food selection is sample sized and you purchase tickets to try the samples. One of the best things to do in Denver on a warm mid-July weekend.
Two Ponds National Wildlife Refuge, Arvada (always free)
Two Ponds is a very small National Wildlife Refuge which was created as a place for migrating birds to stop and rest.
Clear Creek History Park, Golden (always free)
Clear Creek History Park in downtown Golden is now free to walk through, sunrise to sunset every day.
Santa Fe Art Walk (first Friday monthly)
Denver's Art District on Santa Fe Drive features one of the more diverse collections of galleries in the United States, all situated within a short walking distance.
Shell Gas Two For One Ski Ticket
For 2014, Shell gas stations are offering 2 for 1 vouchers at several ski areas with a 10 gallon gas fill up. This means that if you buy at least 10 gallons of gas, you get a coupon good for a free lift ticket if you purchase one full price adult lift ticket. The deal is good at several ski areas.
Unfortunately, full price adult lift tickets have become so expensive that this isn't much of a "deal". But if you just want to ski one day at a participating resort this year, this will get you on the slopes for less than full price.
In Colorado, 2014's participating resorts are:
Copper Mountain
Crested Butte
Loveland
Monarch
Powderhorn
The SkiFreeDeals.com web site has a full list of participating Shell stations and ski resorts.
Learn more at skifreedeals.Com:
Howelsen Hill in Steamboat Springs (always low cost)
Howelsen Hill Ski Area is a historic ski area and Olympic training grounds operated by the city of Steamboat Springs, and is one of the only ski areas left in Colorado that is affordable on a daily basis. This ski area is a decent place for downhill skiers and snowboarders to make some quick laps, and features a nice terrain park, but it is especially nice for Cross Country / Nordic skiers.
There are only a handful of downhill runs served by 1 rope tow and 1 chair lift, but there are miles of Nordic trails. Howelson is lit for night skiing and often stays open until 8pm.
Oh yeah, the huge ski jumps are off limits to the public but they are an amazing sight!
2012 - 2013 Rates:
Lift Tickets: $20 per adult, discounts for children and seniors.
Terrain Park Only: $10
Nordic Trails Only: $10
Children's Magic carpet learning area: Free! (see concession booth for a free ticket).
$10 night skiing tickets are also available Tuesday through Friday.
Full lift ticket pricing breakdown for 2012-2013:
Hours and Season:
CLOSED MONDAYS.
Tuesday 1:00 pm – 6:00 pm
Wednesday 1:00 pm - 8:00pm
Thursday 3:45 pm - 8:00pm
Friday 10:00 am - 6:00 pm
Saturday and Sunday 10:00 am – 4:00 pm
Closed on Christmas Day and special holiday hours are in effect through Jan 3.
Howelson Hill Home Page and Maps:
Steamboat is fairly remote for a day trip, but if you can't afford the lodging in the area, there is a year-round campground that is reasonable called Steamboat Campground.
The Littleton History Museum (always free)
The Littleton Museum is a 14 acre indoor and outdoor history museum depicting Colorado pioneer history. Two historic farm sites featuring a schoolhouse and blacksmith shop bring history to life in a way that consistently rates this museum as one of the top living history museums in the United States.
Patsy's Candies Tour in Colorado Springs (always free)
Patsy's Candies in Colorado Springs takes the making of chocolate candy to a whole new level. Free tours of their factory demonstrate the handcrafted nature of Patsy's Candies and highlight the artisanship present in their high quality candies.
Liftopia Ski Discounts
Colorado resorts offering substantial Liftopia ski discounts:
Arapahoe Basin.
Copper Mountain.
Durango.
Loveland.
Monarch Mountain.
Ski Cooper.
Sunlight.
Winter Park.
Some other resorts have discounts on Liftopia that really aren't discounts at all.
Check out Liftopia:
Rentals
If you need to pick up rentals on your way to the mountain, my favorite super cheap rental shop is Maison De Ski in Idaho Springs.
Children's Museum of Denver Free Days (first Tuesday monthly, 4-8pm)
One of the best free Denver attractions for kids, the Children's Museum of Denver's free day takes place from 4pm to 8pm on the first Tuesday of every month, known as Target Tuesdays.