The Gunpowder Treason and Plot,Thanks to FB friend Maureen for reminding of this anniversary. More information and the full poem can be found at Wikipedia, though I have to say, as gripping as the opening lines are, the remainder is a disappointment.
I know of no reason
Why the Gunpowder Treason
Should ever be forgot.
As a related aside, I watched V For Vendetta earlier this year, and found it much more moving than I expected.
Followup: Steve Denton and Kevin McIntosh also caught this... thanks for the reminder, all!
Another Followup: CSM has an interesting article about Guy Fawkes Day as seen in the US.