UPDATE (10th July, 2010):
JS-Kit (Echo) is now a completely paid service. Free trial is available only for 30 days and after that you have to pay $10 for every 30 days to use their service. As Echo is presently being many big corporate sites like Wshington Post, New York Times and Technorati so I would recommend this to only those who are really serious about blogging and for the rest – better go for the free services viz. Disqus and IntenseDebate…
So here are three new comment-form providers. Select any of these and make your blog completely professional. Presently in Blogger blogs, with custom domain and customized templates, comment-form is the only left element through which people can recognize yours as a free blog, so once you implement any of these new comment-forms you will have a complete pro-blog.
And here are the screen-shots of the comment-forms (in the same order as above):
JS-KIT (Demo Blog)
IntenseDebate (Demo Blog)
Disqus (Demo Blog)
The reason for selecting JS-Kit over Disqus or IntenseDebate is of course the more number of features and facilities you get at JS-Kit.
NOTE: By using any of the above comment-systems, your new comments will not be indexed by Google. So consider these points before you switch to any of these services.
If Google is NOT your primary (or important) traffic source, and even if it is the primary source of traffic, but you have sufficient keywords in your post-titles and content then you may add these forms.
BUT if Google is your primary source of traffic and most of the search queries that bring traffic to your blog is because of the keywords in your comments, then I would suggest you to carry on with your present comment system.
To check and track your visitors source/keywords read this post:
Track your blog’s visitors.
*UPDATE: According to JS-KIT, their comments are indexable by Search Engines, so consider the above “NOTE” only if you planning to use IntenseDebtate or Disqus.
IMPORTANT: Before you modify your template, make sure you back up the present the template by downloading it. And in case you ever want to switch back (from IntenseDebate or Disqus to Blogger Comment system, then make sure you reverse the settings back to “New Posts Have Coments” before you publish any new post).