averageradical

Non-technical summary: Track incoming referrer and write it to any hidden form field with the identifier “referrer” (this can be created in Contact Form 7, for example; see below). Also if the WordPress site redirects from HTTP to HTTPS, this plugin also overrides the redirect to add the referrer to the https URL.
Technical details: This plugin runs client-side JavaScript in every page in HEAD and if there isn’t a session cookie with the name “referrer”, then it writes a session cookie with the name “referrer” and the value of either document.referrer, or if that’s empty, the URL-encoded value of window.location.href. There is another client-side JavaScript in every page before which writes the value of the “referrer” cookie to any INPUT with the ID “referrer”, if it exists.
When integrating with Contact Form 7, add a hidden input field such as the following:
<pre><code>[hidden referrer id:referrer]
</code></pre>
And then add to the Mail’s Message Body:
<pre><code>Referrer: [referrer]
</code></pre>
If you’re just integrating this with HTML, just add this to any form:
<pre><code><input type="hidden" id="referrer" name="referrer" />
</code></pre>

track-incoming-referrer

Track incoming referrer and write it to any hidden form field with the identifier "referrer".

Phoenixheart

This plugin displays a greeting message to visitors that come from different urls (known as referrers). For example, you may want to welcome Diggers with a message that reminds them to digg your story, or you may want to ask Del.icio.us users to bookmark your post, and so on.
Not only about a cool greeting box, it greatly helps to more efficiently interacts with your readers and build a better relationship between you (your website) and them.
<h3>Features</h3>
<ul>
<li>A pure AJAXed admin interface with elements tidily divided into tabs. Entries can be added/modified/(de)activated/deleted without the need to reload the entire page, or even with just one click. Starting from 3.2, there are even bulk actions!</li>
<li>The greet box position can be set to before, after, or (starting from 3.2) both before and after the post. A <a href="http://www.phoenixheart.net/2008/11/referrer-detector/#custom_tag" rel="nofollow ugc">custom tag</a> can also be used in case you don’t want to show greetings on every post.</li>
<li>Users can also put a tag (<code><?php if (function_exists('referrer_detector')) referrer_detector(); ?></code>) into the template to show the greet box anywhere of choice.</li>
<li>Excluded URLs can be specified in order to hide the greet box from specific users (like ones from Google Reader).</li>
<li>A * wildcard character can be used in both referrers and excluded URLs. For example: google.* will apply for all localized Google domains.</li>
<li>Multiple URLs can be merged into one entry, seperated by commas. For example: del.icio.us,delicious.com.</li>
<li>Ability to include WordPress attributes like url, title, author, category etc. into the greetings.</li>
<li>A STAT panel to get data reporting about the visitor statistic.</li>
<li>Many installed-by-default entries (which can be restored anytime with one click). Upon installing, you will have these entries available:
<ol>
<li>9 rules</li>
<li>Bing</li>
<li>BuySellAds</li>
<li>Del.icio.us and Delicious.com</li>
<li>Digg</li>
<li>Gizmodo</li>
<li>Google</li>
<li>Lifehacker</li>
<li>Live Search</li>
<li>Reddit</li>
<li>StumbleUpon</li>
<li>TechCrunch</li>
<li>Technograti</li>
<li>TechRadar</li>
<li>Twitter</li>
<li>Yahoo!</li>
</ol>
</li>
<li>Compatible with caching plugins, since the core is written and run in JavaScript which is normally not cached.</li>
<li>Automatically detect user’s country and display the localized greetings (if any).</li>
<li>Ability to backup and restore entries, excluded URLs, and options, very convenient for transferring between sites.</li>
<li>Welcome box now can be shown in lightbox style using <a href="http://www.ericmmartin.com/projects/simplemodal/" rel="nofollow ugc">SimpleModal</a> jQuery plugin.</li>
<li>Optimized OOP code allows extreme readability and easy modifications for other plugin developers.</li>
</ul>
<h3>History</h3>
<ul>
<li>4.2.1.0</li>
</ul>
<ol>
<li>Temporarily removed Backup/Restore feature for security reasons.</li>
</ol>
* 4.2.0.3 
1. Add Bing and BuySellAds entries. You can either reset the entries or create on your own using the new icons inside referrer-detector/images/icons/ directory 
1. Deleting the plugin now removes all options and data – Don’t try it 😉 
1. Several fixes and enhancements 
* 4.2.0.2 Raise compatibility 
* 4.2.0.1 
1. Added an option to display the welcome box in light box style 
1. Fixed several IE bugs 
* 4.1.0 
1. Welcome box CSS rewritten 
1. Added ability to optionally display related posts 
* 4.0.1 – A hot fix attempting to fix allow_call_time_pass_reference problem 
* 4.0 
1. Completely rewritten code – now optimized OOP 
1. Data are now stored in database for better security 
1. Introduced the ability to backup entries, excluded URLs, options (you can choose from these 3) 
1. Introduced the ability to add localized messages to welcome user visiting from different countries. 
1. STATs reporting is now using Google Chart 
* 3.2 
1. Introduced Bulk Actions 
1. Supported wildcard (asterisk character) in both referrers and excluded URLs, for example google.* matches all Google domains. 
1. Introduced the ability to merge several URLs into one entry, seperated by commas. For example: del.icio.us,delicious.com 
1. The greeting box can now be place BOTH before and after a post. 
1. Some minor graphic changes and bug fixes. 
* 3.1 
1. Fix the javascript bug on IE that prevents the greeting box from being shown up. 
1. The javascript files are now <a href="http://www.crockford.com/javascript/jsmin.html" rel="nofollow ugc">minified</a> instead of <a href="http://dean.edwards.name/packer/" rel="nofollow ugc">packed</a> to improve performance. 
* 3.0.2 Made a mistake in 3.0.1 
* 3.0.1 Fixed the bug that made the home page XHTML invalid 
* 3.0 Added the Stats panel. Now the plugin can show (customizable) stats of visits from different referrers from time to time. 
* 2.2.0.1 Fixed the javascript bug when jQuery library is not included by default. 
* 2.2 Added ability to include search terms into the greet box. Supports almost all common search engines. 
* 2.1.1 A small fixed on form serialization and a (possible) security flaw. 
* 2.1 A minor release, introducing the ability to restore the entries to default. 
* 2.0 Second major release, which has some important changes: 
1. A much much clearer admin interface with tabs, along with some enhancements. 
1. An option to not show the greeting box for visitors that come from specified URLs (“excluded” ones), for example Google Reader. The list can be edited easily via admin panel. 
1. Users can now put a tag (<code><?php if (function_exists('referrer_detector')) referrer_detector(); ?></code>) into the template to show the greet box. 
1. Some javascript files are packed to save bandwidth and loading time. 
* 1.2 Several important fixes and improvements. 
1. Fixed the bug when multiple greet boxes are shown if the page is filled with multipe full posts (such as homepage). 
1. Fixed the bug in the initial SQL data where the default RSS feed url is set to mine (tooo bad I forgot to check it). 
1. Fixed the javascript error when the page has no referrer. 
1. Update the installation and FAQ section. 
1. And some little stuffs… 
* 1.1 Better support to work with cache plugins. Now the generic options take effect without the need to delete the cache. 
* 1.0.1 A quick fix for caching problem (if cache is enabled and the first user has no referrer, the plugin will fail). 
* 1.0 Initial version

Referrer Detector

Helps your blog detect where the user comes from and automatically displays the corresponding greetings.

track-incoming-referrer vs Referrer Detector