Hoot Suite ~ http://www.hootsuite.com/
Have more than one twitter account? This tool lets you manage several accounts from the same place. It also compresses links, schedules your tweets, and tracks your stats.
Tweetdeck ~ http://www.tweetdeck.com/
Twitter organizer. Use the Tweetdeck Dashboard to manage your followers, put them in group, see your conversations, and watch your favorite conversations through your specified search queries.
Bit.ly ~ http://bit.ly
Compresses links and tracks click through stats.
Tiny url ~ http://tinyurl.com
Similar to bit.ly
Tweet Stats ~ http://tweetstats.com
Get the stats. See who you and also your followers' tweet and reply to most and at what time.Trending stats.
TwitPic ~ http://www.twitpic.com
Like to use photos with your twitter? Use your Twitter user name and password to log in and upload photos directly to your feed or with a link.