Apple has released OS X 10.10, Yosemite. It’s available for free from the Mac App Store, and I highly recommend you install it. Yosemite brings a much-needed graphical update to OS X and offers a slew of new features.
However, the download is large (about 6GB) and if you immediately proceed to the install, the download is deleted afterwards. If you pause and create a USB drive for Yosemite before you install it, you’ll never need to wait for that download again.
Here’s how to do it: http://lifehacker.com/how-to-burn-os-x-yosemite-to-a-usb-flash-drive-1647137212