To get SQL Server 2005 Developer Edition, you need to be enrolled in the beta, or be an MSDN Subscriber. In either case, aside from milestone builds (e.g. Beta 3), you will also need a fat pipe to download them, because they're only offered online. Closer to RTM you will probably be able to order a CD, but until then...

You can get a preview version of SQL Server 2005 Express Edition either by itself or with a Visual Studio Express beta product.

However, before you go hog wild and download and install everything you can find, note that there are some serious limitations with which versions of which product can co-mingle on a single machine. You will have to remove previous versions of the .NET Framework and all SQL Server 2005 programs before installing the December CTP. Do not try to intermix this release with Beta 2, the October CTP, or any other IDW. Mix and match like CTP products only! This goes for SQL Server and Visual Studio products. If you need some help removing previous versions, see the following link:

http://weblogs.asp.net/wallym/archive/2004/10/17/243703.aspx

The Visual Studio December CTP has not been released yet, so I strongly advise you to avoid trying to mix the Visual Studio bits currently being offered at betaplace with either the Developer or the Express editions of SQL Server 2005 that are currently being offered on the same page. I also recommend avoiding side-by-side installations with SQL Server 2000 if you can at all avoid it. And be sure to read the CTPNotes.mht document that is included in the image of whatever edition you decide to download and try out.

These are beta products, and you should take care to install them on machines that can we wiped at any point, or else use a virtual machine. And please view these comments by Kent Tegels and Andrew Watt before you go any further:

http://sqljunkies.com/WebLog/ktegels/archive/2004/12/06/5413.aspx

http://www.tfosorcim.org/archives/000293.html

Please read over the above text, and the two links I have included, at least once before continuing. After you have done that, you can go here for the SQL Server Express December CTP products (Express Edition itself, as well as Express Manager).

http://lab.msdn.microsoft.com/express/sql/default.aspx