I have a number of sites I frequent and do business with, but I'm sure there are plenty of great ones that I don't know about.  I'm interested to hear about the ones that you visit often.  Here's my list:

1.  Books, CDs, and sometimes other stuff:  Amazon.com

2.  Audio books:  Audible.com

3.  Movie rentals:  Netflix.com

4.  Online music:  Napster.com, MSN.com, Walmart.com

5.  Hardware reviews:  Tomshardware.com

6.  Online auctions:  Ebay.com

7.  Electronic payments, fund transfers:  Paypal.com

8.  Computer hardware:  Newegg.com

9.  Web email:  MSN.com, Yahoo.com

10.  Search:  Google.com, MSN.com

So, here are mine.  Let me know what yours are.