I've got an MSDN subscription; I'd like to play with Windows Azure; I don't particularly want to pay (more) for it. I'm not looking for 5-nines uptime.
Is there any way to avoid being Slashdotted when using Azure? Can I set limits for usage, so that my site is unavailable rather than getting charged?
YES - just use spending limits. see http://azure.microsoft.com/en-us/pricing/spending-limits/
(this wasn't available at the time the question was first asked)
You also have the ability to set up billing alerts in Azure, so you can keep track of when you are approaching a certain amount of spend - https://azure.microsoft.com/en-us/documentation/articles/azure-billing-set-up-alerts/
No, there's no simple way of setting usage limits with Azure (or most other cloud providers). If you go over your allotment they'll charge you.
For compute, this should be easy, as you set the site to run a certain number of instances and it's not going to go above that without you doing some work.
For storage, you are left to your own devices to implement quotaing.
For network and storage transactions (if you directly expose your storage to the Internet), you are pretty much at the mercy of the Internet.
If you'd like to try it out for a month with no risk of overages and no credit card required, go to http://windowsazurepass.com/. There are a lot of promo codes floating around, one of them is CloudCover (for the Cloud Cover Podcast).