DB2 Express-C - Free to Develop, Deploy, Distribute

The DB2 "Express" version is now completely free. I think it is a response to Oracle Express.

"DB2 Express-C is a version of DB2 Universal Database Express Edition (DB2 
Express) for the community. DB2 Express-C is a no-charge data server for 
use in development and deployment of applications including: C/C++, Java, 
.NET, PHP, and more. DB2 Express-C can be run on up to 2 dual-core CPU 
servers, with up to 4 GB of memory, any storage system setup and with no 
restrictions on database size or any other artificial restrictions."

2 dual-core and up to 4GB memory, this is good enough for most production use. For more information, please look at: DB2 Express-C Overview. Unfortunately, it is not as free as open source software.

Please note that the current production version is UDB 8.2 which are free to use. 9.0 (viper) version is still in "test drive" stage, and requires a "Try and Buy" license which lasts for 90 days.

This article helps you understand the architectural differences between, MySQL, Postresql and DB2.