Animals
Ptarmigan
Pika
Golden Eagle
Moose
Bighorn Sheep
Marmot (aka "Ice Age Guinea Pigs")
Elk
Mountain Goat
Black Bear
Bobcat
An earlier version of this page listed this as a Mountain Lion. I've seen a number of them since (always at night) and I'm quite sure it's Bobcat and not a Mountain Lion. There's no obvious long, powerful tail and he/she appears to be fairly small. There are no spots and the ears appear rounded, but the real tell is the eyes. Bobcats tend to have a white perimeter (as in the picture below) and mountain lions have a very dark perimeter.
Insects
'Cow Killer' Ant (Mutilidae)
Mating Moths
Wildflowers
I'm still working on identifying these. If you know what they are, please let me know.
Common Trees in Colorado: csfs.colostate.edu/colorado-trees/colorados-major-tree-species/