Make sure that you have the official Nvidia drivers installed, not a 3'rd party driver.
Open a terminal, Enter the following;

sudo gedit /etc/X11/xorg.conf

In the window that open up, look for:

Section "Device"

And within that section, add:

Option "Coolbits" "5"

As an example, mine looks like:

Section "Device"  
     Identifier     "Device0"  
     Driver         "nvidia"  
     VendorName     "NVIDIA Corporation"  
     BoardName      "GeForce GTX 560 Ti"  
     Option         "Coolbits" "5"  
 EndSection

Now restart your system, and open the Nvidia X Server Settings by typing the following in a terminal:

sudo nvidia-settings

Go to the "Thermal Settings" section, and you now have a slider to adjust your fan speed, just like in Precision.
 
Disclaimer: Running your fan speed too low can cause problems, Running at a high speed can cause wear. I am not responsible for anything you do to your product. Please also read the disclaimer that pops up the first time you enable fan control.

pagelm

awesome.  Does this apply for non-nVidia products?  Is "coolbits" a package that needs to be installed?

 
Unfortunately coolbits is limited to nVidia drivers only as it is a portion of the drivers that you install. I know of no similar package or source that does the same thing using the mesa drivers.

Brilliantly simple and articulate. Thanks.

Zammy

Thanks for this OMG, I just changed to Mint and I couldn't get nvclock to work with it, nice solution as my 295 card runs pretty warm under X

 
It has been my experience that running nvclock under Mint will cause a full system lockup and require a reboot.

VanishingImage

I can't find the "Device" section when the window pops up.  Im running Ubuntu 12.04 LTS. Trying to get Steam to work but so buggy it runs on my patience quick lol. Ive tried Play On Linux but a little lost on how install a game using a CD

 
If there is no device section, then X may not be configured properly, or you may not even be running the NVIDIA driver yet.
 
If you're not sure whether you have the NVIDIA driver yet, you can install it in terminal with:
 

sudo apt-get install nvidia-current

 
After you do this, reboot. Once the system is up and running again, open the Linux version of the NVIDIA control panel:
 

sudo nvidia-settings

 
Look for the option to save the xorg file or to save the configuration data, and save it to /etc/X11/xorg.conf
 
Then follow the rest of the steps in the first post.