A lot of people have been asking about availability for the Commerce Server 2007 Developer Edition. The good news is that it is now available. And, we're giving it away too.
No, you did not hear that wrong - it is a free download for all. No expiration.
The restriction is that this is only for evaluation and development - you cannot use it for production systems. (So, for the environments where business data is changed or analyzed, data is staged, and the production Web farm - you need Standard or Enterprise licenses.)
http://www.microsoft.com/commerceserver/evaluation/software.mspx has the download.