Free universe tool monitoring all database
Not that I'm fond of it, but if you use Nagios, it's rather easy to add some database-related plugins. You could do worse than have a look at:


PostgreSQL: Very useful. The latest versions are available at Bucardo.
MySQL: quite a few are available, make your pick. 
Oracle: but you'd better ask the Oracle experts.
MsSQL: I'm not using it yet but will test it soon.