Educated writers (according to a small and not random survey of my acquaintances) think myriad is an adjective and should only be used as such:
He had myriad reasons for wanting to graduate early.
Myriad birds rose from the lake at dawn.
Captivate lets you import myriad graphic formats onto a slide.
But the most I can find any authority saying about it is this from Bryan Garner in the Oxford Dictionary of American Usage: "The more concise phrasing involves using this word as an adjective, not as a noun." He says it is "better" to use myriad as an adjective, but does not go so far as to say that using it as a noun is actually wrong.
The Greek root word for myriad is defined both as "countless" and as "10,000."
If you think of myriad as meaning "countless," you have to use it as an adjective:
We have countless toys no longer used by our children. [you can't say "a countless of"]
We have myriad toys no longer used by our children.
But if you take the underlying Greek meaning of 10,000, then you can use myriad as a noun, but it makes more sense to add the s, like this:
Dozens of cars were parked in the small lot.
Thousands of cars were manufactured in our town.
Myriads of cars packed the interstate. [Tens of thousands of cars...]
Webster's defines myriad as either 10,000 or "a great number," and scoffs at those of us still thinking it is wrong to use myriad as a noun, and then says this:
The noun myriad has appeared in the works of such writers as Milton (plural myriads) and Thoreau (a myriad of), and it continues to occur frequently in reputable English. There is no reason to avoid it.
So, I guess we sticklers for the adjective can lighten up a little--it is acceptable to use myriad as a noun. Sigh.