Frédéric Descamps of Percona.

Percona Toolkit is Maatkit & Aspersa combined. Opensource and the tools are very useful for a DBA.

You need Perl, DBI, DBD::mysql, Term::ReadKey. Most tools are written in Perl, and whatever is in Bash is being re-written in Perl. There is also a tarball or RPM or DEB packages.

Know your environment. The hardware & OS are crucial for you to know. How much memory/CPU do you use? Do you use swap? Is this a physical/virtual machine? Do you have free space? What kind of RAID controller? Volumes? Disk? What about the network interfaces? What IO schedulers are used? Which filesystem is the data stored on? To answer all that, just use pt-summary.

Know your MySQL