You could also download the My Wallet add-on for Firefox.

This means you don't need to download the java-script from their servers each time (which may be compromised by a hack).
Only 790 people have downloaded this amazingly.

https://addons.mozilla.org/en-US/firefox/addon/my-wallet/?src=api


For a little additional security you could run it in Firefox portable and keep it on a USB stick away from your normal browser.

And use watch only addresses for large amounts of Bitcoin - with the private key secured on a paper-wallet or equivalent