IM - Instant Messaging

Adium

Connect to multiple IM accounts simultaneously in a single app, including: AOL IM, MSN, and Jabber. Beautiful, themable interface.
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Email

Mozilla Thunderbird

Powerful spam filtering, solid interface, and all the features you need.
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RSS

Vienna

Very nice, native RSS client.
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RSSOwl

Solid cross-platform RSS client.
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Peer-to-Peer Filesharing

Cabos

A simple, easy to use filesharing program. Gnutella network.
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Transmission

Very nice torrent downloader.
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FTP / SFTP

Cyberduck

Very good all-around ftp program.
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OneButton FTP

Simple, fast FTP, with a focus on "advanced queuing".
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Fugu

Not as polished as Cyberduck, but has a side by side interface showing your files and the remote files.
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HTML / Text Editing

Smultron

Text / html editor. Integrates with Cyberduck for editing websites.
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Nvu

Visual tool for making websites (WYSIWYG html editor), comparable to Dreamweaver.
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IRC

Colloquy

Great IRC client.
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Note Taking

Freemind

An innovative application for note taking and outlining. Takes time to get used to, but some people swear by it.
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MSN Chat

More Filesharing

Tomato Torrent

Download torrents. Nice and simple.
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del.icio.us Client

Cocoalicious

Let's you easily post to del.icio.us.
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