Today comet C/2012 S1 ISON will pass closest to Earth. No worries here, close is relative in space and in this case means a comfortable 0.43 AU (64,200,000 km or 39,900,000 miles) from Earth. The comet should be easily visible well up in the dawn sky as an unaided eye object in northern Hercules.
Update: Yes, this post was written and scheduled well before the comet disintegrated at perihelion. If there is anything left it is closest to the Earth today. Many amateurs, large scopes, even Hubble have attempted to recover the remains. No one has reported finding anything.