Buy.com (especially with free shipping specials and coupons)perhaps not always the lowest price; but excellent service after the sale, easy web site to use
Mwave.com (not always the lowest price; but excellent service after the sale, easy web site to use)
Newegg.com (less of a selection; but still a mwave.com clone)
Hypermicro.com (for SCSI stuff)
Office depot (B&M: for when I need it now, right now)
AT Forum (of course, for used hardware/software)