If you have some hardware with an I2C, SPI or 1 Wire interface connected to your Raspberry Pi you need to enable the kernel modules that drive these interfaces.
The Raspbian Wheezy images from 2012-10-28-wheezy-raspbian.zip onwards contain the necessary kernel modules but they are not enabled by default. You must edit a couple of configuration files and install the i2c-tools package first.
You need to remove the 'blacklist' entries from raspi-blacklist.conf.
sudo nano /etc/modprobe.d/raspi-blacklist.conf
You need to comment out the two blacklist entries as above. Save and exit from the editor (ctrl O and ctrl X).
Next you need to add the i2c-dev and 1 wire modules to /etc/modules.
sudo nano /etc/modules
Add the lines 'i2c-dev', 'w1-gpio' and 'w1-therm' as above. Save and exit from the editor (ctrl O and ctrl X ). Install the i2c-tools package.
sudo apt-get install i2c-tools
You should now reboot your Raspberry Pi
sudo shutdown -r now
Once rebooted you can detect i2c devices attached to your Raspberry Pi (if any).
sudo i2cdetect -y 1
If you have a Revision 1 board you will need to use
sudo i2cdetect -y 0