On October 24, 2007 Comet 17P/Holmes went from having a brightness of magnitude 14 (extremely dim, only visible with a powerful telescope), to a magnitude 2.8!
That's an increase in brightness of about half a million times! It's visible in cities with bright lights, during a very bright moon!

For a history of Comet Holmes, I direct you to Gary W. Kronk's Cometography page.

Here is a diagram from my copy of TheSky showing the orbits of Comet Holmes and the planets.

As you can see, Comet Holmes is actually moving away from the Earth. It is very unusual for a comet to get brighter as it moves further from us... normally they get brighter as they come closer to the sun. This is due to ice and dust being blown off of the surface of the comet by the solar wind.
But for some reason, Holmes has demonstrated a sudden burst of brightness. Astronomers are still debating what caused it.
Some think it might be due to a rich vein of volatile ices on the comet's nucleus that was suddenly exposed to sunlight. This is actually the second time that Holmes has flared up in this manner, the last time coming in its discovery year of 1892.

Update 11/15/07: Scientists say that the coma of Holmes is now larger in diameter than the Sun!

Update 02/15/08: Here's a photo-collage from Astronomy Picture of the Day showing how the coma of the comet has grown in size over the last 3 months.