Just a short reminder that the final 2010 Estimated Tax Payment is due January 18, 2011 (since January 15 falls on a Saturday and Monday, January 17 is a public holiday in the US). To avoid (further) penalties for underpayment of estimated tax, in general, by January 18, 2011 you should have paid (in combination with 2010 US taxes withheld and all other 2010 Estimated Tax Payments) the lower of:
1) 90% of your (eventual) 2010 tax liability, or
2) 100% of your 2009 tax liability (110% if your 2009 Adjusted Gross Income exceeded $150,000 or $75,000 if Married Filing Separately)
Note: You do not need to make estimated tax payments if your final 2010 tax liability does not exceed $1,000.
You can find the form and instructions at: http://www.irs.gov/pub/irs-pdf/f1040es.pdf