Almost everything other than produce. I have an Amazon credit card too. I think there will only be a year or two before they start having to charge tax though. Our country is too far in debt to let them continue their tax loophole practices. So take advantage of the savings while you can!
Are you in the United States? Sales tax is collected by the state rather than federal government. Some states already collect sales tax on it (very few though). Enforcement seems to be the difficult problem because the state has no authority to enforce anything like this across state lines so they actually have voluntary compliance.
When I was in ohio I read the state tax form it said to list what I bought through mail/online and guestimate the sales tax on it. I think hardly anybody complies with this.
I hope they don't use this as another excuse to increase federal powers :-\