Buttercups (Ranuculus species) have five shining yellow petals. Most are a deeper gold than this one and have fuller petals.
Nuttall's larkspur (Delphinium nuttalli), a native wildflower up here in the Rockies. Beautiful but deadly. As are all larkspurs, and all delphiniums. No need to worry, as long as you don't eat them.
This monkshood is our wild species (Aconitum columbianum), which ranges from the Rockies to the Pacific Ocean. It's not as flowery as those you can buy at nurseries or through catalogs. But it's just as beautiful.
I always feel like I've found a treasure when I discover monkshood blooming along the creek—though I admire from a distance.
Individual plants of monkshood, as well as other members of the "friendly" Buttercup Family, vary greatly in their content of toxic aconitine, but I'm not taking any chances. While I love learning about toxic plants, I'm not about to use myself as a guinea pig.
By the way, perhaps that word "aconitine" made you wonder about winter aconite, the super early blooming bulbs with the yellow flowers? Yep. Them, too.
Don't get freaked, though—many, many plants are poisonous...including daffodils :)
As long as you don't eat them (or, in some cases, rub them on your skin), you can enjoy them in your garden without worry. I sure do, and I haven't been poisoned. Yet ;)