Our favorite, of course, was the 2005 story about a downturn in the maple syrup market causing Vermont's maple trees to explode as a result of sap build-up. Other gems include a story about steroid use in orchestras, portable "vanity" zip codes, and advertisements that could be projected onto the moon. Here's a list of and links to these and more April Fool's Day stories broadcast by NPR.
Even Performance Today has gotten in on the act. Last year, Fred Child and company aired a piece about a new concert hall in Copenhagen built entirely out of Legos.
Be careful out there today!