Generally catalog (on-line, phone or by mail) retailers are required to collect sales tax if they have a physical presence in the same state as the purchaser, based on your billing address usually. Otherwise you get into issues of taxing interstate commerce which can be dicey, and why there is occasionally a move in Congress to tax internet sales.
Is there an Apple Store in your state? You'll get taxed in the app stores or other online purchases from Apple.
Does Google have offices in your state? If not, they probably won't collect tax.
Amazon has generally been trying to dodge collecting taxes but my understanding is they generally will in states where they have offices or warehouses.
btw, even if the retailer isn't collecting sales tax, law in your state may mean that you should be declaring a use tax as part of your tax return each year.