If you're on the West Coast like me, you may still have time to use up your health care flexible spending account by going to an enterprising optometrist, dentist, or chiropracter. If you've spent more than (or very, very close to) 7.5% of your gross income in health care expenses this year -- which is a LOT of health care expenses, by the way -- you might want to pay a couple more bills to take advantage of those expenditures.
The easiest way to create deductions in the last few hours of the year is, of course, charity. Give until you can't give any more, and most charitable institutions allow you to use your credit or debit cards to pay online, and it will count for 2007, right up until midnight. If you must send money to your church, community food bank, or other small non-credit-accepting institution, run! Bike! Drive! (careful! It's a party night!) to the nearest post office that postmarks until midnight, and stick a check in the mail.
Finally, if you're self-employed or have a small business whose expenses you write off on your taxes, and you use the cash method of accounting, now is the time to pile on those profit-reducing costs! I make a small income photographing knitting and designing knitting patterns, so I have been struck with the unavoidable compunction to go buy yarn for my next creation. If you don't have any shops nearby open, turn again to the web. Do you need books? Amazon.com's still open. Paper? Ink for your printer? You get the idea.
Note that small incomes like mine are only considered self-employment if you make a profit in one out of every three years. Otherwise, it's a non-deductible hobby.