Oracle is jumping on the free database bandwagon: Oracle Database XE is a free (as in beer) Oracle database. This parallels Microsoft's SQL Server Express and of course all of the open source databases. The press release indicates that XE will be limited in both RAM (1 Gb) and storage (4 Gb). Microsoft's earlier free database, Desktop Edition, was unfortunately also limited in how many concurrent activities it would support, and made it just a toy.
Keep in mind, XE is still Oracle 10g under the hood, so it's going to have a big footprint, the download is 150Mb! Harry Fuecks has some details on building a PHP site on top of it.