Free Database Versions: IBM, Oracle and Microsoft

Oracle announced the general availability of the free version (Oracle 10g Express edition) of their database product following microsoft(SQL Server Express edition) and IBM (DB2 Express-C). This product supports Oracle Call Interface for C and C++, Oracle HTML DB, ODBC, Oracle Data Provider for .NET, OLE DB, JDBC, and PHP development environments. The following are a few features of the product:

1. Available on 32-bit Linux and Windows Installs using native installers
2. English (single byte character set) and International (Unicode) versions available with support for 10 major languages
3. Supports up to 4GB of user data
4. May be installed on a server with any amount of memory, but will only use up to 1GB RAM of available memory
5. Fully upgradeable to other Oracle Database 10g editions
6. Oracle Text for efficient text-based searches

An Oracle Java developer's guide is also provided along with an Oracle 10g express edition getting started guide. Although all three come with a free to develop, deploy and distribute licenses, they have a few restrictions. The following is a list of restrictions for the free version of databases provided by IBM, Microsoft and Oracle.
Oracle 10g Express edition

• Processors: Can be installed on multiple CPU Server but will use only ONE CPU.
• Memory: A maximum of 1GB memory will be used (Even if you have more).
• Database size: Up to 4GB user data.

Microsoft SQL Server Express edition

• Processors: 1 CPU, but can be installed on any server
• Memory: 1 gigabyte (GB) addressable RAM
• Database Size: 4 GB maximum database size

IBM DB2 Express-C

• Processors:Upto 2 dual-core processors.
• Memory: A maximum of 4GB addressable memory.
• Database Size: Unlimited

Looking at this, if you are planning on using a free database for you application, it looks like DB2 Express-C is the clear will with respect to database size and memory availability.