What is the state of the art -- in terms of scalability, management, monitoring -- for people who need to keep their data in house? Maybe X where X:FathomDB as Xobni:Gmail? Or are we stuck hiring a DBA?
Think of it in the context of building a database-backed enterprise app that customers run in house. You don't want to ship a DBA to every customer...
And if your time is free, then you are probably a startup and you can get SQL Server and bunch of other MS tools for free via http://www.microsoft.com/BizSpark/
MySpace runs on SQL Server: http://www.sqlservercentral.com/blogs/steve_jones/archive/20... and if it's good enough for them it will likely be good enough for your site as well. I'm not sure if the scale-out story is any good (there are "partitioned views" which are basically "sharding" but I never used those), but scale-up story is as good as they come - it can efficiently utilize 64-cores of the HP superdome.
I don't know much about Oracle and IBM DB2, but I know that the latter has a free version without too many restrictions on it, so give them a spin as well. If ease of use is importnat to you, common wisdom has it that Microsoft's is the easiest to use among the top commercial vendors. NB: I did not verify the common wisdom, YMMV :-)
PostgreSQL is pretty much on par in terms of features and performance with SQLServer and Oracle. So if you're looking for a free solution to roll your own then look no further.
The only reason I can see for going with a commercial DB is when you not only want commercial support (you can buy that for Postgres, too) but a shrinkwrapped "don't bother me"- solution with SLAs etc. Be prepared to pay through your nose for that, though.
In terms of usability there is no big difference between any of the commercial or non-commercial offerings. They all have their special peculiarities (which is why you always hire a DBA who is familar with your RDBMS, not just with "any" RDBMS) and the shiny GUIs that some of them provide are not normally of interest in day-to-day work.
This is also the reason that databases like Couch DB or Amazon Simple DB or Google's App Engine are so appealing. They all have the promise of reducing db management headaches, at the cost of sacrificing features.