Plagiarizing Shakespeare

I volunteer at the local library by setting up their display case.  During this month of April, the theme is Shakespeare, more specifically: "The Play's the Thing."  In researching Shakespeare on the Internet for ideas of what to include in the display, I was astonished to learn that Shakespeare wrote 38 plays, 154 sonnets, and introduced some 3,000 words into the English language. 

Then I was doubly astounded to find numerous quotations are still being quoted from his works, like: neither a borrower nor a lender be, to thine own self be true, it was Greek to me, and the course of true love never did run smooth.  Centuries have come and gone, but The Bard's words endure.  I wonder if there is a statue of limitations on plagiarism . . .