1) Those offering expanded search/grouping/collaborative (or other interactive) functionality.
2) “Pre-tweeting” capabilities 
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3) Apps allowing for greater capabilities as far as posting from sources other than via a user’s own Twitter page (see: @woork’s AWESOME post “Twitter: send message from a PHP page using Twitter API” for one example ….)
4) Alternative/even user-customizable display methods for Tweets (check out ExecTweets for one example of a really cool, animated tweet display method [looks like uses jQuery/Ajax]….). I plan on adding an app like this one, but one that will allow for the scrolling of tweets to stop upon the user’s placing the mouse over a given tweet that interests him/her, a la the Twitter custom Search Widget, which reminds me of a whole other area I plan on playing on with at some point in the future .
OK, enough Twittering around, back to business (and remember my fellow tweeples to focus on end goals/results first, and then on developing the best methods toward reaching those goals!) Stay cool everybody, and please feel free to contact me for ideas, collaborations, feedback, etc.