I've been using PostgreSQL 8 in beta/RC for a while and it's pretty good, and now it's released - woo-hoo!
We also use rhdb/PostgreSQL 7.3.x on a lot of boxes and that's fairly schweet. Not that I've anything against MySQL (I'm pretty sure that it's obvious that I prefer the GPL over BSD for the licence at least) but it just isn't anywhere near as featured as PostgreSQL.
Don't even mention SQLite! ;-)