Seesmic Desktop is a desktop client app that resembles TweetDeck's first cousin because the feature set is largely the same, but with significant differences in layout and functionality. Seesmic also is written in Microsoft Silverlight, as opposed to TweetDeck's Adobe Air, which makes Seesmic available across multiple platforms. (A web-based version also exists with much of the functionality of the desktop client.) Seesmic also differs from TweetDeck in that the former has a plug-in architecture, and much of the program's functionality is not baked directly into it but, rather, supplied by a gamut of add-ons.
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