[snip] There are a number of good relational databases that play on multiple platforms, none are free! You may not pay a license fee for some, but you will pay in other ways. From personal experience, consider the true multi-platform aspect and what it is worth to you. Then consider the cost of migration, both technical and administrative. And don't forget the cost of converting the data.
I did a large conversion from MS-SQL to DB/2. The database was spread over mainframes, Windows, Unix and Linux platforms. It worked well when done, and still does today. The conversion took three people a year and almost two months by 24 hours per day to migrate the data.
A large commercial database is good if you what the reliability, support and availability of trained professionals to convert and maintain the system. My SQL is good and I've used it for some jobs, however I've had a number of problems that trace back to its origins. I've also used Oracle, Tamino and some pretty custom stuff. Each has its place. I do like Tamino for its native XML abilities, Oracle for its scalability and DB/2 for its robust implementations and inter-operability with mainframes. Not one is easy.