I installed the latest version of WP 2.3.1 onto my server. However, I can not find the “Top Posts Widget”. Would you please tell me from where I can download the “Top Posts Widget” which is the one using in wordpress.com now ?
Since you are self-hosted, you are using different software than we are here at wordpress.com and we know little about the software you are using. Post your question over at http://wordpress.org/support/ as they know all about WP 2.3.1.