I've read good things about postgresql (http://www.postgresql.org/about/). I don't know why postgresql is not more popular. Portability is going to be a major concern when I do my next rewrite. While postgresql runs on many platforms, it does not seem to have the installed base of MySQL.
mysql is 'easy' to pick up, but lacks many of the features that are actually useful for writing dependable software. it is very easy to write completely non-portable, inefficient, sloppy bad software with mysql - and never be able to move to some other backend without extreme pain. postgresql is a little further up the learning curve, but there's quite a bit more power exposed to the programmer/dba. and you're a lot closer to "standards-compliant" - which makes your support folks very happy. --elijah