Not only were there already free open-source DBMSs such as MySQL as well as related toolkits like Hibernate, but most recently (in late 2005 and early 2006) the three major vendors in the DB arena themselves came out with no-charge versions of their key commercial database products.
Here's a summary of their new free products. This summary is also posted at http://asiapac.com.au/Links/Database.htm (Australian server) and http://notestracker.com/Links/Database.htm (U.S. server).
THE database phenomenon of 2005 / 2006 -- feature-laden, industrial-strength FREE versions from the leading DB vendors!
It could be a while before I have the the time to install and try out any of these three well-respected DB products, but I'm really looking forward to doing so.
There are many things to be considered, and I merely note here that the IBM DB2 Express-C offering does not restrict a database's size to 4 GB and this alone would differentiate it as the product of choice for those databases which you expect will need to store more than 4 GB of data. (This could easily be the case, for example, when there are lots of BLOBs.)