Oracle XE is good, free and still pretty good but you are limited to small(ish) amounts of data. You can do most of your testing and dev with it though. There a

Oracle XE is good, free and still pretty good but you are limited to small(ish) amounts of data. You can do most of your testing and dev with it though. There are a number of options such as Standard One which offer pretty cheap licenses for most of the functionality you'd need to compete with Postgres and MSSql. That said, I've had great experiences with Postres and MSSql too, it's just that I learnt all my core db stuff from Oracle 7 and moved on from there. I haven't bothered with mysql for some time. I think I'd always go for Postgres in preference. MSSQL and Oracle have many advantages when working in coporate/enterprise environments and you probably won't have to worry about licensing costs so much then!