Still in beta as of this writing, Slipstre.am is designed to provide additional filtering to Twitter streams apart from lists or hashtags. It's not quite a full-blown client, but rather a kind of meta front end for Twitter that lets you extract that much more signal from the noise. The interface strongly resembles the classic Twitter layout, though it's missing a few recent common features, such as URL shortening. What's important about Slipstre.am is not the interface itself but its filtering system, which can work by criteria other than just text strings.
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