There are many 3rd party commenting systems. By 3rd party, I mean these that are not developed specially for blogs or WordPress.
With all due respect to bloggers, not everything boils down to WordPress in terms of blogging. Sure, WordPress is amazing platform, but commenting is not strictly just for blogs.
As I mentioned above, not everything starts and ends with WordPress, but many business websites might look into opportunity to add a bit of interaction on the main business and even item/product pages.
This is not a review article, nor comparison between different systems.
Well Known External Blog Commenting Systems
Probably these are some of the most popular 3rd party extensions/plug-ins or parts of code that can provide your website with extra functionality.
I am not going into details of features and capabilities of these, but just list the downside. Technically you don't have much control over the comments. Other downside is that I personally don't know any way to migrate comments to another system, if you are using these, but you are on the save side in terms of SPAM, do-follow links and number of footprints left behind.
Holly Trinity Commenting Systems
Many bloggers, actually have decided to replace native WordPress system with one of these 3, this is a smart turn in terms of reducing the size of database and reducing SPAM. Most top website and blogs have implemented Discuss, InstantDebate or Livefyre.
All three offer similar features, as well as integration is nearly identical. All claim that this can increase traffic, interaction and returning visitors. This is quite possible, especially if you don't already use commenting system in your website.
Main downside, in addition to inability to migrate to another system later is that bloggers rarely comment on non-native WordPress commenting system.
Discuss, InstantDebate and Livefyre, can be integrated on any kind of CMS, even static website, this is the main plus.
Other Popular CMS
In terms of work on open source CMS, there are many options depending on the platform. There are many and I have to apologize to developers of some of the extension, if I don't mention your creation.
If you are a developer of comment extension, please do not hesitate to leave some details in our comment form below!
The list in huge, absolutely have to mention that 3 years ago, there were one or 2 comment extensions for Joomla, that were barely usable. Today the list of high quality components and modules for Joomla 1.5, 2.5 and v3 is huge. For full list please visit JED (Joomla Extension Directory)
Some of the most notable are:
Drupal another popular CMS, offer some native functionality and modules related to commenting, but there are additional modules that can be easily installed and configured.
In terms of e-commerce comments are much more related to orders and special requests. I think highly recommended for every online store to implement order commenting system, some notable and free extensions are:
Brainvire Order Comment
On the other hand, one good extra is to check for review plug-ins for Magento that can bring more trust into your shop and function similar to blog commenting system, some popular plug-ins are:
Amaze Reviews & Ratings
Detailed Product Review
Social Commenting Systems
Relatively new, two options only for now – Facebook and Google Plus.
There was huge discussion recently which one to use, well both have pros and cons like any other, though there are certain benefits in terms of social media presence.
Add more interaction to your website with commenting system.
Increase number of visitors
Improve interaction with readers/customers
Commenting systems are not just for bloggers
Bloggers, please comment on other systems too!