We've released SQL Server 2008 Express with Tools (a new edition for 2008) and SQL Server 2008 Express with Advanced Services to the web. You can find information comparing the three editions of SQL Express in my recent blog post. You can find links to download any of the three editions of SQL Express from the Express web site. For those of you looking for Management Studio Express, you can install it from either SQL Express with Tools or SQL Express Advanced; there will not be a separet download for Management Studio for 2008.
It's worth writing about the naming of the files and how the file name relates to what you're installing. As in the past, there are different downloads available for Express depending on the OS architecture and language you want to install. SQL Express core has three packages available while SQL Express w/ Tools and SQL Express Advanced have two each.
SQLEXPR32_x86_ENU.exe / SQLEXPR_x86_ENU.exe
SQLEXPRWT_x86_ENU.exe / SQLEXPRWT_x64_ENU.exe
SQLEXPRADV_x86_ENU.exe / SQLEXPRADV_x64_ENU.exe
All these pacakges have prerequisites which are detailed in earlier posts in this blog and on the download pages for each edition. If you have any questions post them in the SQL Express forum on MSDN.