SSH Ubuntu without password

+ From Ubuntu PC
https://help.ubuntu.com/community/SSH/OpenSSH/Keys
Short guide
http://lani78.wordpress.com/2008/08/08/generate-a-ssh-key-and-disable-password-authentication-on-ubuntu-server/
+ From Linux PC
http://www.thegeekstuff.com/2008/11/3-steps-to-perform-ssh-login-without-password-using-ssh-keygen-ssh-copy-id/
+ From Windows PC (using putty)
http://www.howtoforge.com/how-to-configure-ssh-keys-authentication-with-putty-and-linux-server-in-5-quick-steps
More details at Key-Based SSH Logins With PuTTY
http://www.howtoforge.com/ssh_key_based_logins_putty
+ Disable password authentication + Specify Which Accounts Can Use SSH
https://help.ubuntu.com/community/SSH/OpenSSH/Configuring#disable-password-authentication

sudo /etc/init.d/ssh reload

Note: you will be in trouble if you loose your key.