Which SD cards are compatible?
The site http://www.raspberrypi.org/quick-start-guide says:
You will need an SD card with an operating system preloaded before you can boot the Raspberry Pi. A brand-name (not generic) Class 4 card of 4GB or more is recommended. To obtain an SD card image, and for instructions on how to flash an SD card from a Linux or Windows PC, please refer to http://www.raspberrypi.org/downloads.
Which SD cards are compatible with the Raspberry Pi?
Actually all of them are compatible. I am using a *Sandisk 8GB Class 10 UHS-1 SDHC Card* which is the best on the market right now. Well actually you might have problems booting with some SD Cards but check this question on how to fix it.
I just noticed that my SD card is actually Class 2. Oops. It's been working fine so far.
I just successfully activated the latest Raspbian (2014-01-07-wheezy-raspbian) on 64Gb SDHC Class 10 Ultra Sandisk
You have to be careful and refer to the link, because there have been reported issues with people using microSD in SD adapters and also some of the higher class/speed SD cards suffer data transfer rate issues, so I would refer to this link before buying a new SD card.