One of the common needs of a contribution or event registration system is to track what campaign that transaction is destined from and what link originated the transaction. This allows organizations to calculate what links / web sites / advertising / mailings are most effective and use that information to influence their current or future campaigns. Some organizations (e.g. wikimedia foundation) use a three level tracking scheme: Campaign, Appeal and Fund, while other organizations might choose to use only one tracking field.

This issue came up at the San Francisco developer camp. There is no built-in support for this in CiviContribute or CiviEvent. However we can accomplish something very similar using a combination of custom fields, populating fields via a GET url and a civicrm buildForm hook. We've made some changes to v2.2.4 to make it easier. Here is the recipe for tracking either a contribution or event registration page.

1. Create a custom group called "Contribution Tracker" which extends Contribution. Within that group create three custom fields, one each for Campaign, Appeal and Fund. In my test setup i created these as searchable text fields with a length of 64 characters. Lets assume these custom fields have ids of 4, 5 and 6.
2. Add these three fields to a Profile which is included on that contribution page. let assume the contribution page has an id of 1
3. In a drupal module (lets call it civitracker) create the following hook
   

   function civitracker_civicrm_buildForm( $formName, &$form ) {
       // enable tracking feature
       if ( ( $formName == 'CRM_Contribute_Form_Contribution_Main' ||
              $formName == 'CRM_Contribute_Form_Contribution_Confirm' ||
              $formName == 'CRM_Contribute_Form_Contribution_ThankYou' ) &&
            $form->getVar( '_id' ) == 1 ) { // use  CONTRIBUTION PAGE ID here
   
           // use the custom field ID and custom field label here
           $trackingFields = array( 'custom_4' => 'Campaign',
                                                       'custom_5' => 'Appeal',
                                                       'custom_6' => 'Fund' );
           $form->assign( 'trackingFields', $trackingFields );
       }
   }
   

4. Make sure your civitracker module is enabled

In the links that you add to your blog post or website, should pass in the values of the above three custom fields as GET parameters. Thus a sample URL would look like:

http://drupal.demo.civicrm.org/civicrm/contribute/transact?reset=1&id=1&custom_4=Campaign2009&custom_5=Summer&custom_6=General

CiviReport will ship a few reports that will analyze contributions based on Campaign, Appeal and Fund (these are being built for the Wikimedia Foundation). You can read more detailed specifications of the reports on the wiki. Some of the above changes are in v2.2.4. The issues that were filed (and fixed) to get the above working include:

Allow admins to track where contributions / event registrations are coming from via tracking codes
Allow contribution and event fields to set custom values via the URL