Luckily, Macs have a couple of options, specifically USB booting, and since most have an SD card slot, we can use those as well. Not so easy anymore is it? Even downloading the OS from the Mac App Store wouldn’t do when the hard drive needs replacing or the Recovery Partition is corrupt. Installing OS X has never really been a particularly difficult task, but try doing that on a MacBook Air or a system with a broken optical drive. With the advances in technology, faster data access (SSD), and slimming hardware footprints, legacy technologies are typically the first cuts made to get these devices thinner and lighter while making them more powerful and efficient.
Jesus Vigo goes over the steps to create a bootable USB to install OS X 10.7-10.8 and OS X 10.5-10.6, as well as how to put multiple versions on the same USB. How to create a bootable USB to install OS X
