If you are full time or your withholding does not meet a certain level based on your total income you are
required to make ES Payments or
you will be penalized. I believe the basic rule is you are required to make ES payments if you will owe $1,000 or more in tax after subtracting your withholding and credits.
This years payment schedule:
April 15th, 2008
June 16th, 2008
Sept 15th, 2008
Jan 15th, 2009
IRS form 1040-ES explains the requirements:
http://www.irs.gov/pub/irs-pdf/f1040es.pdf
more info:
http://www.webtaxcenter.com/f1040es.jsp