There used to be great charts of the EC2 spot price history at cloudexchange.org, but the domain has expired and the author hasn't responded to an email I sent asking about the site.
Are there any replacements? Does anyone want to maybe fork the site? The code is available from https://github.com/tlossen/cloudexchange.org . I think it could also use some updates to add the newer instance types.
The charts provided by this site are a lot nicer and more responsive than the charts built into Amazon's EC2 console.