Other Commands

Change the system default editor on Ubuntu (yes, I prefer vi over nano):
sudo update-alternatives --config editor

Repair all tables in all mysql databases on a server::
mysqlcheck -u root -p --auto-repair --check --optimize --all-databases

Burn a CD from the command line:
wodim -v -eject cdimage.iso

Rename multiple files with mmv:
mmv "*.jpeg" "#1.jpg"

Convert SSL key to format that Amazon ELB likes:
openssl rsa -in somekeyfile.key -outform PEM

Find all broken symlinks
find / -type l ! -exec test -r {} \; -print

Recursively search an replace text
find ./ -type f -exec sed -i 's/string1/string2/' {} \;
find . -name "*.txt" -print0 | xargs -0 sed -i '' -e 's/foo/bar/g'

Check HTTPS certificate expiration
echo | openssl s_client -connect google.com:443 2>/dev/null | openssl x509 -noout -dates