Mac

To clear out old " open as " enties.

/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister
 -kill -r -domain local -domain system -domain user 

Flush DNS cache

In Lion and Mountain Lion 10.7
sudo killall -HUP mDNSResponder 
 
In Leopard / Snow Leopard 10.6
dscacheutil -flush cache
 
In Tiger 10.5
lookupd -flushcache

Write an iso image from a folder

hdiutil makehybrid -iso -joliet -o image.iso /path/to/source