SQL Server 2008, Developer Edition

Microsoft has done a clever thing with their Developer tools: they've recognized that it's more important to make it easy to put the product in the hands of someone who'll use it than it is to make huge sweeping profits off of every buyer. SQL Server 2008 Developer Edition provides all that you will need to learn and use SQL Server 2008: all that's missing here compared to the full Enterprise Edition are some of the Analytics tools and advanced features that, if you need them, you've already done all you need to do in this copy of the database, anyway (analytics is typically only critical to an Enterprise-level shop, who will most likely have full licensed server versions of SQL Server 2008 Enterprise Edition). 
View Price : SQL Server 2008, Developer Edition
As a database technology, SQL Server 2008 has come into its own, finally being feature-rich enough and capable enough to be compared to Enterprise big-guns like Oracle. Microsoft are also nudging their way into SAP/Business Objects territory, making ancillary activities like Reporting and Transformations much easier to perform. The SQL Server 2008 interface is powerful, it is surprisingly easy to use, and installation and maintenance really do seem to be a breeze. You also get useful features like the SQL Server Books Online to help you learn, integration with Visual Studio to speed up your development time, and some surprisingly simple interfaces for doing powerful transforms/ETL like the Business Intelligence Development Studio.
If you're a developer, and you found downloading and installing the free version(s) of SQL Server 2008 to not meet your needs (or in my case, be fraught with problems), you can't argue with the price. This product works well to get you fairly far towards "guru", and can help teach old Database Dogs new tricks. :) You should be advised that this product is only intended for learning and working in a development environment and should NOT be used to deploy and/or support Production data or applications.