Annoucement link.  "Free to develop, deploy, and distribute" and suited (among other things) for "developers working on PHP, Java, .NET, and Open Source applications."

According to the FAQ, the Express Edition has the following limitations:

• Limited to 4GB of user data
• Only one instance per server
• Only executes on one processor if running on a multi-processor server
  
• Will only utilize 1GB of memory, even if the server has more

Oracle 10g Express Edition comes with a "browser management interface" which I assume is better than Microsoft's MSDE command-line OSQL.

My company is considering Oracle as an alternative product platform, so I'm sure I'll be giving this a try sooner or later.