Twitter Clients:

Tweetvisor: Allowing people discovery right within Tweetvisor

SocialWhale: Providing MrTweet statistics for their users

Notification Apps:

Topify: Providing MrTweet statistics and recommendations to users about their recent followees

NutshellMail: Providing MrTweet statistics to their users to track their recent usage

Firefox Plugins:

MrTweet Addon: Allowing you to get useful context about a user via statistics and recommendations

Vertical Apps:

Tweetminister: Providing a constantly updated list of influential users in UK politics. More info here