From left to right, cherry, maple, oak, walnut. Cherry darkens with age (shown here aged about 2 years) while walnut lightens. There can be a great deal of variation between boards, as well as the display of colors on individual monitors so take these as a guide only. I can mail samples upon request.
Poplar is used mainly as a secondary wood for drawer interiors and nonvisible structural members.
A good resource for other species can be found here.