Tool I use:
HootSuite: I really LIKE this site because I can post to several different pages at one time. Plus schedule tweets. There is so many things you can do!
TweetDeck: This is similar to HootSuite. I say give both of them a try and see which one you like the best.
TwitPic: You can post photos or videos to TwitPic from your phone, from the site, or through email. I use this one for my hubby's airbrush business. I post pics of his artwork and also can see how many views and people can comment on them.
TweetMeme: Retweet cool stuff you come across. The TweetMeme retweet button is for websites and blog publishers who want to encourage their audience to tweet their content on Twitter. The button shows a live count of the number of times your webpage or blog post has been tweeted.
Twellow: A search directory of people by area of expertise, profession or other attribute listed in personal profiles on Twitter. I call this the YELLOW PAGES of Twitter!
WeFollow: Another SEARCH directory like Twellow.
TwitterGrader: I got 98.2 out of 100- not bad! This is a cool site that gives you some info to help you do better on twitter. Can't believe I signed up for Twitter 3 years ago- WOW!
Tweeparties: LOVE LOVE this site! You can see a calendar of twitter parties and also start a twitter party. If you don't know what a twitter party~ well is a fast virtual party on Twitter that uses #hashtags to keep track of the conversation. Most are held at night and sometimes they even give away prizes. I've done a couple and I got a lot of followers through the party. Most of the time a Brand/Company will be the one hosting but anyone can throw a PARTY!
What about you any COOL Twitter sites/tools/apps you use and want to SHARE?