Tweetdeck: A desktop client that allows you to manage all of your Twitter activity without being logged on to the web. This is very popular due to its ability to organize your followers into groups. Tweetdeck has also recently added the ability to update your Facebook status from the interface as well chat with your Facebook friends.
Twellow: The Twitter yellow pages! Find Twitter users by category or geographic area. This is a good way to find new and interesting people to follow.
Tweetie/Twitterberry: For those of you looking for ways to tweet on the go, I prefer Tweetie for iPhone. I have also heard good things about Twitterberry, which is for the Blackberry.
Grouptweet: Group message broadcasting for Twitter. This tool gives you the ability to send private tweets to only a select group of followers which you choose. I’ve seen this used successfully for everything from web development projects and school assignments to diet and exercise accountability clubs.
Monitter: This is one of my favorite tools. It lets you monitor Twitter for a set of keywords and see what people are saying.
Twitterfox: If you use the Firefox browser, this is a nice extension which notifies you when your friends update their status on Twitter.
Twitthis: A way for people to send Twitter messages about blog posts or websites. When you click on the TwitThis button or link, it takes the URL of the webpage and creates a shortened URL. You can then send the shortened URL and a description of the page to others on Twitter. I love using this to quickly share the URLs of interesting articles or posts I come across while browsing the web.
Hashtags.org: Hashtags bring some order to the updates of Twitter users. For example, a couple of years back the hashtag #sandiegofires was used in order for those interested to easily follow this topic. Hashtags.org provides real-time tracking of all the hashtags being used on Twitter.
WhoShouldIFollow: This is a very simple tool which looks at your current friend list and recommends others to follow based on that list. One of the Twitter questions I am asked most often is how to find more people to follow. This makes it quick and easy to do.
TweetBeep: If you want to know when someone mentions your name, company, product or competitors you can get updates sent to you to stay on top of it all.
Twitterfeed: Allows you to quickly tweet your blog posts. Just sign up and enter your blog feed and Twitter login information. When you publish a blog post, it will be sent to Twitter automatically.
Have any others to add?
Christine