This post is incomplete without mentioning the previous one about development tools, so make sure you read it before/after this.
In this post, I'd like to mention two monitoring tools, and one knowledge base.
Two of the tools are made by Quest Software/ToadSoft that make Toad for MySQL. Both of them are Freeware.
A side note: TOAD was born as "Tool for Oracle Application Developers". Because "Tool for Oracle Application Developers for MySQL" sounds weird, TOAD is known officially as "Tool For Application Developers".
Spotlight for MySQL
Spotlight is a tool perhaps more known among SQL Server and Oracle users. It shows you the entire flow of data throughout your database in a nicely laid out executive-summary kind of way. It has lots of colors and it's impressive to look at. When it works. Seriously, I've had a lot of trouble with it, to make it work the way it should too many times. It's also slow as hell, and tends to hang or crash, like other tools by Quest. But, when it does work - it's very nice. Quest released this tool for MySQL as well, and since it's free it doesn't hurt to try it out.
Knowledge Xpert for MySQL
This is basically a well laid-out knowledge base, of what Quest Software thinks you should know about MySQL. Full of useful examples, and clear explanations for things that the MySQL manual doesn't mention. I recommend installing it and taking a look, you might find something interesting. It was helpful to me when I started learning MySQL.
Warning: practically this is just one big help file, but Quest decided to bundle an annoying interface, a slow installer and one of those load-on-startup-take-memory-do-nothing-system-tray-icon which you need to get rid off after installation.
MONyog is a small program with a web front-end which can monitor several MySQL servers and supports some customizable metrics. I won't go into detail on the good and bad things in MONyog here (I plan to write a review of it later on), but I do suggest you try the trial version. MONyog is not free, and doesn't have a free or community "lite" version.
At the time of writing the latest stable version is 1.52, but 2.0 beta 2 is already available and brings additional features like historical reports and better graphs.