Your readers can preview the comment before submitting. Ajax Comment Preview let you type in a comment and click “Preview”. The information you enter is sent back to the server via the snazzy XMLHttpRequest Javascript object. The server then decides how to format that information (where part of this decision is the filtration of the information through WordPress’ content filtration system - the real advantage of this method). The newly formatted information is then sent back to your browser where you can see exactly how your comment will look once you submit it.

If your browser doesn’t have all the clever bits necessary for all this, fear not. You can forego the comment previewing and just submit the comment as usual.

Pricing: Free
Requirements: WordPress 2.1+
Source: http://blogwaffe.com/ajax-comment-preview/