Spendbitcoins is probably the best for buying anything online with Bitcoins. Very easy to use.
Love love love Spendbitcoins. Used it many times now to convert to Amazon gift credit. The first time, Jeremy was being dicked around by Amazon, which caused a long conversion delay, so he sent me an additional, generous gift credit, for the inconvenience. With Amazon gift credit, you bank it in your account and it won't expire before you can spend it. If you're concerned about losing market value, or hacking if you don't keep an offline wallet, convert it! Can't wait until the conversions are instantly automated.