The MiniDrive (Nifty competitor) (flush SD adapter for Macbooks) - $6 (if funded)
The MiniDrive sits flush and fits securely into the SD slot of all modern (2010+) MacBook computers.
WITHOUT THE MINIDRIVE:
If you just stick any old SD card into your MacBook slot to add storage, it's going to stick out of your computer. That is not something viable for a device that's thrown into your MacBook case or backpack. It will get broken...
It's a knockoff of the Nifty drive. But this is much more barebones and as such cheaper. If you don't know about the Nifty drive (http://www.kickstarter
Its $6 via Indiegogo.com. Campaign ends in 11 days. If it gets funded, you get a $6 MiniDrive. If they don't reach the goal, you get your money back. Seems like a win-win situation to me...
If you want one asap - they have stores on Amazon and eBay (roughly $20 on those sites, Amazon slightly cheaper)