Plant Identification Website?

Does anyone have a good website to help me identify plants and flowers as they come into bloom?  You know, so when we’re out on our nature walks and one of them say, “Oh, look at that.  There are some flowers.  What kind are they?” that I could answer with something other than, “It looks like a weed with a pretty white flower to me.”  I was thinking something like whatbird.org or the arbor day site for trees, but along the flower/plant lines.