A few years back many of us would have thought that a computer maker was crazy for not including an optical drive on a notebook. Today with most netbooks lacking them, not having an optical drive is common.
If you use a MacBook and rarely use the optical drive and want more storage OWC has a new product called the Data Doubler you will be interested in. The device is a bracket that replaces the optical drive with a SSD or HDD.
Any drive 9.5mm tall can be used in the bracket. Full instructions and the needed hardware for the install are included. OWC makes the brackets to fit just about every MacBook there is for $79.99. Once installed the Mac OS will recognize the drive and you can use it just like you do the normal HDD.