The one that really surprised me was USB cables. I needed a USB cable on Friday night. I checked Best Buy and the best they could do was something like $36 for the length I wanted.

I then found that I could buy THE SAME cable from MacMall for $6. Not a generic brand--the exact brand name cable. Same manufacturer's part number and everything. And I could get overnight FedEx with Saturday delivery by 10 AM for $10. So, from MacMall the net result would be half Best Buy's price for exactly the same thing, provided I could wait about 14 hours for delivery.

(I ended up not buying from either of them, as a more thorough search of my cable drawer turned up the cable I needed).

For USB cables, here's what I now recommend to people. I think this advice applies to other cables, too, such as DVI and HDMI and network cables:

Someday, you are going to find that you need a cable and don't have the one you want on hand. And you'll find you don't want to wait for overnight delivery, so you are going to end up going to Best Buy and spending way too much for your cable.

So why not take a measure now to make sure that does not happen? Go to some place like monoprice.com, and give yourself a budget of, say, 75% of what one long cable would cost you at best buy. Use that budget to order a variety of cables. For example, for USB, for the cost of one 10 or 12 foot Best Buy cable, you could get 2 or 3 long USB cables from monoprice, 2 or 3 six footers, and a handful of 2 or 3 footers. Do that now. No rush, so use the cheapest shipping.

Now you've got a stock of cables, and as you use them you can replenish your stock at your leisure. Even if you end up never using many of the cables you will be ahead as long as this saves you from a single Best Buy cable purchase.