Essential MySQL tool No. 3: xtrabackup and xtrabackup-manager
If your databases are in use every day, all day, giving you no "overnight" during which tables can be locked for backup, xtrabackup is your solution. Also known as Percona XtraBackup, this tool performs nonblocking backups and is the only free, open source tool that can do this. By comparison, proprietary nonblocking backup software can cost more than $5,000 per server.
xtrabackup also offers incremental backups, allowing you to back up only the data that has changed since the last full backup. Adding incremental backups to your backup process is powerful, given the reduced performance hit of these tremendously smaller backups.
Furthermore, another project has grown up around xtrabackup that makes managing a full backup plan even easier: xtrabackup-manager. Although this tool is new and still in development, it holds a lot of potential because it offers advanced features like rotating backups with groups and backup set expiring. Together, xtrabackup and xtrabackup-manager are a formidable and free backup solution.
More info: http://www.percona.com/docs/wiki/percona-xtrabackup:start | https://launchpad.net/percona-xtrabackup
Essential MySQL tool No. 4: tcprstat
tcprstat is probably the most esoteric of the 10 on this list. The tool monitors TCP requests and prints statistics about low-level response times. When you become familiar with the response time way of thinking about performance, the payoff of tcprstat is significant.
The principle is elaborated in the book "Optimizing Oracle Performance" by Cary Millsap and Jeff Holt, and it applies equally well to MySQL. The basic idea is that a service, in this case MySQL, accepts a request (query), fulfills that request (execution time), and responds with results (result set). The service's response time is the time span between receiving a request and sending a response. The shorter the response time, the more requests can be served in the same amount of time.
Parallel processing and other low-level factors play a significant part here, but the simplified upshot is that there are 28,800 seconds in an 8-hour workday, so reducing response times by just four-tenths of a second (from 0.5 to 0.1 second) results in 230,400 more requests served each day. tcprstat helps you achieve this.