Can you use a US debit/credit card to buy stuff on foreign websites (in their currency)?

The first two answerers have good points. I can't add much other than be aware what country you are ordering within. Here in the US we don't worry too much about Canada but, if its any other country you should do your homework first.

Yes you can, however buying things on websites can be risky. The best sites use secure payment options eg paypal google checkout etc

i generally steer clear of sites that don't offer this security because without this secure option you are are basically giving all your banking details to a stranger.

Hope this helps

The best thing to do would be to make a Paypal account if you have not already, then register your card and that'll be the only time you'll have to use your card's number, after that it's done through Paypal and it does not require the card's number again, other than that you can use the card by itself, but it's highly recommended to use Paypal or another service of the same nature.