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.
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 :-)