BitAddress is one of the most popular paper wallets, that allows to print private keys and Bitcoin addresses onto paper. A private key and public key can be generated by randomly moving you mouse around the screen. Than, the keys can be imported to another wallet system (via the WIF key) or printed out for keeping offline as a cold storage.
For better security, it is recommended to unplug the Internet access while the keys are being generated as well as to wipe the Internet history after the keys have been created. Ideally, they'll be generated on a brand-new computer to completely avoid any malware interference.