Find Tweeters like You 

Directories and User Search Tools

Just Tweet It
        A twitter directory sorted by interest
Twubble
        This tool will automatically find people who are compatible with  your interests. However, if you follow too many people, then Twubble may recommend some people that you already follow
Twittie Me
        Search for similar users and advertise your twitter page
Twitdir
        Search for words in usernames, locations or descriptions
We Follow
        Follow people through popular tags
Twellow
        Find people by area of expertise

Track Twitter Users 

Find out who the top Twitter users are

Twitterholic
        Find out who the top twitter users are
Toptweet
        Find out the top twitter users on one place.
Twitrank
        a list of the top 150 Twitterers

Twitter Links

Work with lnks on Twitter

Twitter Buzz
        See the most popular links on Twitter
Link Bunch
        Share more than one link on Twitter
Tweetwire
        Sorts out links posted on Twitter into news format
Twitterlinkr
        Find out the latest links posted on Twitter
Twitturls
        Find out the latest URLs posted on Twitter
Twitter Split
        Change the way you post links by adding your domain on the beginning of the link

Find out what is the latest trends on Twitter

Sites that tell the latest stories and trends on Twitter

Twitscoop
        A search tool that tracks trends as well as certain events.
Trendistic (previously Flaptor Search)
        A search tool that can be installed on your browser.
Tweet Scan
       Search keywords on Twitter
Twitterment
        Search Twitter users and topics
Tweetmeme
       find out the latest Stories on Twitter
Twistori
        Find out what twitter loves, hates, believes, etc.
Twitturly
        Find out the most talked about stories on Twitter
GoodBadMe
        Keep track of the good and bad on the twitterverse
Favotter
        Find out the most favored (saved to favorites) tweets

Track certain keywords on Twitter

See keyword behavior on Twitter with these sites

Tweet Beep
        Find out who is talking about you or your website through certain keywords
Site volume
        Enter five keywords and see their activity on Twitter
Tweet Volume
        A more personalize version of Site Volume
Monitter
        Monitor Twitter conversations on three keywords. Good for catching the latest news
Hashtags
        Track a certain keyword on Twitter
Twemes
        See twitter memes or tags for Twitter
Tweetchannel
        Find out what people are talking about through certain keywords called channels.
Twitter Meter
        Find the trends of certain keywords
Flaptor Trends
        Compare the trends of three keywords on Twitter
Twitter Spectrum
        Find out the dominant keywords via a tagcloud generated.
Serendipitwiterrous
        Search for tweets of a certain person using certain keywords
Twittertroll
        real-time Twitter search engine