The answer is, it depends.  Do you have latex or oil based paint?
 

Latex paint is the easier of the two.  Many waste companies will accept latex paint if it is dried.  This is true for the City of Longmont.  You can leave the paint open until it dries, or to speed the process, you can add Oil-Dri or kitty litter to the can.  Once the paint is dried, it can be put out with your regular trash for pick-up. 

Oil based paint is more difficult.  These products should be taken to your local Hazardous Waste collection site.  In Boulder County this is the Household Hazardous Waste facility at 5880 Butte Mill Road in Boulder.

Also, the Habitat for Humanity ReStore at 1351 Sherman Drive in Longmont will accept good, useable paint and stain.  Visit their website for more information.