Monitor MySQL activity per database?
I try to stay away from having multiple databases in one instance. Instead I have multiple instances set up on a server where each one is dedicated to a database.

MySQL Cacti templates are a good monitoring solution. If you are enterprise customer then MySQL Enterprise Monitor works. Also check out mycheckpoint from openark.

EDIT
Here are some tutorials for setting up multiple instances on one server:


Running multiple instances of MySQL on the same machine
How-to: Setup Multiple MySQL Instances


You could also use MySQL Sandbox as it automates a lot of different set ups for you.