I will be continually updating this list of homeschool classes and supplements in Colorado.  Please contact me if you know of any other classes of interest to homeschoolers throughout the state of Colorado.
Homeschool Supplements

Secular Options

Options Schools (various locations throughout Colorado)

Creekside Collective (Boulder County)

Religious Options

Olive Tree School (Fort Collins)

Homespun Academy (Loveland)

Masters Hand (Longmont & Loveland)

Classes for Homeschooled Children

Homeschool Science Colorado (organizes science field trips and Science Olympiad teams)

Rocky Mountain Bird Observatory Homeschool Programs

The Teaching Company maintains a list of current cultural events in major cities throughout the U.S., which may be of interest to homeschoolers.  Go to their home page, linked above, and click on the "Your Account" button on the top right side of the page.  If you don't have an account, create one.  Once you do, you will get a page that gives you links to cultural events in the city or cities you select.

Chess Mates (Fort Collins chess club; not just for homeschoolers)