A list of free, open-source software for Mac OS X.

Web Browsing

Mozilla Firefox
The premier free, open-source browser. Tabs, pop-up blocking, themes, and extensions. Considered by many to be the world’s best browser.
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Camino
Based on the same Mozilla foundation as Firefox, Camino is a more native OS X app. Fast and simple.
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Video Player / Video Podcasting

Democracy Player
Beautiful interface. Plays any video type. Subscribe to video RSS, download, and watch all in one. Torrent support. Search and download from YouTube and others.
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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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Podcasting

Juice
Solid podcasting client.
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Video Playback

VLC
Plays more video files than most players: Quicktime, AVI, DIVX, OGG, and more. Pretty good interface.
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MPlayer
The interface and buttons are not as nice or as Mac-like in feel, but it may play even more video files than VLC.
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iPod Ripping

Senuti
Get music off of your iPod.
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iPodDisk
Copy music off your iPod in the finder, like a regular disk drive.
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Handbrake
DVD to MPEG-4 ripper/converter.
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Burn
CD and DVD Burner.
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Word Processing / Office Suites

NeoOffice
Built on OpenOffice.org. Big, full featured suite of tools for word processing and spreadsheets. Compatible with Microsoft Word documents and OpenDocument Format.
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AbiWord
A word processor. Leaner and quicker than OpenOffice. Compatible with Microsoft Word documents and OpenDocument Format.
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Sound Recording

Audacity
Simple sound recording tool.
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Graphics, Photo, Publishing

Seashore
Based on Gimp, but doesn’t require X11.
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Xee
Fast image viewer and image browser.
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Gimp
Photo editing application that rivals Photoshop in features (requires X11).
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Inkscape
Vector graphics application (requires X11).
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Scribus
Desktop publishing. Complicated install, but a nice app.
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FTP / SFTP

Cyberduck
Very good all-around ftp program.
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Fugu
Not as polished as Cyberduck, but has a side by side interface showing your files and the remote files. Some people prefer it.
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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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Credits: http://www.opensourcemac.org