Microsoft recently posted two new versions of SQL Server 2008 Express Edition. One is called SQL Server 2008 Express with Advanced Services, which includes a basic version of SQL Server Management Studio (SSMS), Integrated Full-Text Search (iFTS), and the ability to run Reporting Services reports against local databases. They also have SQL Server 2008 Express with Tools, which only includes the basic version of SQL Server Management Studio (SSMS).
Both of these still have the same 4GB database size limit, along with the one CPU and 1GB of RAM limit that regular SQL Server Express has had since the 2005 version of SQL Server Express. If your machine has more than one CPU or 1GB of RAM, you can still install and use any of the Express Editions, but SQL Server will only use one CPU and 1GB of RAM.
The nice thing about SQL Server Express is that it is free and it is completely compatible with its big brother editions of SQL Server. You can start out on Express Edition and migrate to something like Standard Edition when you need to, very easily.