Resellers are free to offer their own promotions. Apple also offers its own. You'll find a few promos that are nearly always ongoing available through Apple, the main one being a $100 rebate when one of certain specified printers is purchased with a computer. There are also sometimes discounts on .Mac when purchased with a computer, and sometimes you can get software discounted as well.

If you have a student, faculty or staff member of an educational institution in your family you are eligible for the education discount. This may take a little bit more off the price of the refurb offers. (Keep in mind though that refurbs don't always qualify for the other discounts/rebates at the Apple Store web site... check the rebate terms and conditions carefully.)

Additionally, several employers have discount programs with Apple, as do federal government employees.

The Apple Store does charge sales tax based on ship-to state. Shipping is free for orders over $50.

Check with your local Apple Retail Stores. I believe the Genius Bar will transfer all files from an old Windows computer to a newly purchased Mac at no charge for Switchers. I don't know if you have to buy the machine there or if you can bring in a recent purchase.