Twubble - recommends Twitter users to follow, it generates a list of users displaying which people you already follow are subscribed to these users.
GroupTweet - send a private message blast to a group of people.
You have to create a new Twitter account, tell all members to “follow” the new account, then any member can direct message that account, which in turn will be sent as a direct message to all members.
If you want a more on-the-flly tool, something where your blast message is a one off thing, try Twitter Groups.
Twitterfox - get a stream of updates (recent, replies, messages) and publish tweets from a browser extension near your system tray
Twitterlocal - get a stream of tweets from people around a location (enter a city, state, or postcode), here’s one for where I live in Perth - “6000″ “20 miles” - it gives you a webpage, RSS feed, XML, and offers a desktop app…others are TwitterWhere, another TwitterWhere, and @locals.