Are you over the optical drive? No worries, you're not alone. But if you happen to be low on disc space, there's more than a few ways that'll allow you to take advantage of your unused DVD eating hardware, sacrificing it for the addition of a second hard drive. Sounds interesting? Read on!
Other World Computing (OWC) just recently released a nice little kit that helps folks wit a Mac Mini or iMac to get a little more hard drive space by replacing the optical drive with an internal HDD slot. Unfortunately, it looks like their Data Doubler ($80) only fits on Mac Mini's from early and mid 2009 and iMac's 20-inches and up from the middle of 2009 onward.
You can also follow the DIY route via iFixit's tutorial upgrade and see if you can pull it off without the need of a constructed kit.
Many have stated that having an internal hard drive is almost always faster than external USB. We're not so sure if that holds true anymore with the introduction of eSATA and USB 3.0, but having a portable form factor has always been very important in our book, so kicking out the optical for more data storage is more than welcome.