3M Command system is for damage free hanging of pictures etc. The hooks are very pricey but you can buy the velcro based systems in larger quantities at a better price. This allows you to convert the velcro based system to a hook based system.

It is quite useful because the velcro allows you to make small adjustments to the height of your hooks when lining up multiple pictures.

I bought "Command Small and Medium Picture Hanging Strips value pack" from Amazon