NaPoWriMo, or National Poetry Writing Month, is an annual project in which participating poets attempt to write a poem a day for the month of April.
NaPoWriMo was founded in 2003, when poet Maureen Thorson decided to take up the challenge (modeled after NaNoWriMo, or National Novel Writing Month), and challenged other poets to join her. Since then, the number of participants has gotten larger every year, and many writers' organizations, both national and local, organize NaPoWriMo activities.
Need more information? See the Wikipedia entry for NaPoWriMo!